No system tray detected

Since upgrading to Ubuntu 9.04 from 8.10, I get an HPLIP error saying No system tray detected on this system.
Sometimes, but not every time, I cannot print even if I reboot. Other times printing works fine.

Printer model is HP LaserJet P1005.

---------------
| SYSTEM INFO |
---------------

Basic system information:
Linux martin-ubuntu 2.6.28-11-generic #42-Ubuntu SMP Fri Apr 17 01:57:59 UTC 2009 i686 GNU/Linux

Distribution:
ubuntu 9.04

HPOJ running?
No, HPOJ is not running (OK).

Checking Python version...
OK, version 2.6.2 installed

Checking PyQt 4.x version...
OK, version 4.4.4 installed.

Checking for CUPS...
Status: scheduler is running
Version: 1.3.9
error_log is set to level: warn

Checking for dbus/python-dbus...
dbus daemon is running.
python-dbus version: 0.83.0

------------------------------------
| COMPILE AND RUNTIME DEPENDENCIES |
------------------------------------

note: To check for compile-time only dependencies, re-run hp-check with the -c parameter (ie, hp-check -c).
note: To check for run-time only dependencies, re-run hp-check with the -r parameter (ie, hp-check -r).

Checking for dependency: CUPS - Common Unix Printing System...
OK, found.

Checking for dependency: CUPS DDK - CUPS driver development kit...
OK, found.

Checking for dependency: CUPS devel- Common Unix Printing System development files...
OK, found.

Checking for dependency: DBus - Message bus system...
OK, found.

Checking for dependency: gcc - GNU Project C and C++ Compiler...
OK, found.

Checking for dependency: GhostScript - PostScript and PDF language interpreter and previewer...
OK, found.

Checking for dependency: libcrypto - OpenSSL cryptographic library...
OK, found.

Checking for dependency: libjpeg - JPEG library...
OK, found.

Checking for dependency: libnetsnmp-devel - SNMP networking library development files...
OK, found.

Checking for dependency: libpthread - POSIX threads library...
OK, found.

Checking for dependency: libtool - Library building support services...
OK, found.

Checking for dependency: libusb - USB library...
OK, found.

Checking for dependency: make - GNU make utility to maintain groups of programs...
OK, found.

Checking for dependency: PIL - Python Imaging Library (required for commandline scanning with hp-scan)...
OK, found.

Checking for dependency: ppdev - Parallel port support kernel module....
OK, found.

Checking for dependency: PyQt 4- Qt interface for Python (for Qt version 4.x)...
OK, found.

Checking for dependency: PyQt 4 DBus - DBus Support for PyQt4...
error: NOT FOUND! This is a REQUIRED/RUNTIME ONLY dependency. Please make sure that this dependency is installed before installing or running HPLIP.
To install this dependency, execute this command:
sudo aptitude install --assume-yes python-qt4-dbus

Checking for dependency: Python ctypes - A foreign function library for Python...
OK, found.

Checking for dependency: Python DBus - Python bindings for DBus...
OK, found.

Checking for dependency: Python devel - Python development files...
OK, found.

Checking for dependency: Python XML libraries...
OK, found.

Checking for dependency: Python 2.3 or greater - Required for fax functionality...
OK, found.

Checking for dependency: Python 2.2 or greater - Python programming language...
OK, found.

Checking for dependency: Reportlab - PDF library for Python...
OK, found.

Checking for dependency: SANE - Scanning library...
OK, found.

Checking for dependency: SANE - Scanning library development files...
OK, found.

Checking for dependency: scanimage - Shell scanning program...
OK, found.

Checking for dependency: xsane - Graphical scanner frontend for SANE...
OK, found.

----------------------
| HPLIP INSTALLATION |
----------------------

Currently installed HPLIP version...
HPLIP 3.9.2 currently installed in '/usr/share/hplip'.

Current contents of '/etc/hp/hplip.conf' file:
# hplip.conf. Generated from hplip.conf.in by configure.

[hplip]
version=3.9.2

[dirs]
home=/usr/share/hplip
run=/var/run
ppd=/usr/share/ppd/hpijs/HP
ppdbase=/usr/share/ppd/hpijs
doc=/usr/share/doc/hplip-doc/HTML
icon=no
cupsbackend=/usr/lib/cups/backend
cupsfilter=/usr/lib/cups/filter
drv=/usr/share/cups/drv

# Following values are determined at configure time and cannot be changed.
[configure]
network-build=yes
pp-build=yes
gui-build=yes
scanner-build=yes
fax-build=yes
dbus-build=yes
cups11-build=no
doc-build=yes
shadow-build=no
foomatic-drv-install=yes
foomatic-ppd-install=no
foomatic-rip-hplip-install=no
internal-tag=3.9.2.49
restricted-build=no
ui-toolkit=qt4
qt3=no
qt4=yes

Current contents of '~/.hplip/hplip.conf' file:
[last_used]
device_uri = hp:/usb/HP_LaserJet_P1005?serial=BB0E8YN

[installation]
version = 3.9.2.49
date_time = 04/30/09 20:49:14

-------------------------------
| DISCOVERED PARALLEL DEVICES |
-------------------------------

No devices found.

--------------------------
| DISCOVERED USB DEVICES |
--------------------------

  Device URI Model
  -------------------------------- -----------------
  hp:/usb/HP_LaserJet_P1005?serial HP LaserJet P1005
  =BB0E8YN

---------------------------------
| INSTALLED CUPS PRINTER QUEUES |
---------------------------------

HP_LaserJet_P1005
-----------------
Type: Printer
Installed in HPLIP?: Yes, using the hp: CUPS backend.
Device URI: hp:/usb/HP_LaserJet_P1005?serial=BB0E8YN
PPD: /etc/cups/ppd/HP_LaserJet_P1005.ppd
PPD Description: HP LaserJet P1005 Foomatic/hpijs-ZJS (recommended)
Printer status: printer HP_LaserJet_P1005 is idle. enabled since Thu 30 Apr 2009 20:45:00 BST
error: Required plug-in status: Not installed
Communication status: Good

----------------------
| SANE CONFIGURATION |
----------------------

'hpaio' in '/etc/sane.d/dll.conf'...
OK, found. SANE backend 'hpaio' is properly set up.

Checking output of 'scanimage -L'...

No scanners were identified. If you were expecting something different,
check that the scanner is plugged in, turned on and detected by the
sane-find-scanner tool (if appropriate). Please read the documentation
which came with this software (README, FAQ, manpages).

---------------------
| PYTHON EXTENSIONS |
---------------------

Checking 'cupsext' CUPS extension...
OK, found.

Checking 'pcardext' Photocard extension...
OK, found.

Checking 'hpmudext' I/O extension...
OK, found.

Checking 'scanext' SANE scanning extension...
OK, found.

-----------------
| USB I/O SETUP |
-----------------

Checking for permissions of USB attached printers...

HP Device 0x3d17 at 002:003:
    Device URI: hp:/usb/HP_LaserJet_P1005?serial=BB0E8YN
    Device node: /dev/bus/usb/002/003
    Mode: 0666
getfacl: Removing leading '/' from absolute path names
# file: dev/bus/usb/002/003
# owner: root
# group: lp
user::rw-
user:martin:rw-
group::rw-
mask::rw-
other::rw-

-----------
| SUMMARY |
-----------

error: 2 errors and/or warnings.

Summary of needed commands to run to satisfy missing dependencies:
sudo aptitude install --assume-yes python-qt4-dbus

Please refer to the installation instructions at:
http://hplip.sourceforge.net/install/index.html