Ubuntu 10.04 reverts to metacity from compiz on logout

What's gconf-editor saying?
/desktop/gnome/session/required_components/windowmanager = compiz

It says compiz (after logging in when it doesn't have desktop effects running, and after I turn them on)... so maybe it is running compiz but with visual effects turned off? To be more specific, I had thought it was reverting to metacity because "None" is always selected in the Visual Effects tab of "Appearance Preferences" whenever I log in. If I click on "Normal" to activate effects it spends about 30 seconds "Searching for available drivers" after which the screen flashes (as though it's re-starting the windows manager) and then effects are activated. I click on "yes" when it asks if I want to keep these settings. Where are these preferences stored?

Seems to be (again) a nvidia issue.
What's the output of:
compiz --replace

Compiz settings are also stored in gconf-editor and can be editet with CCSM.
package: compizconfig-settings-manager
https://help.ubuntu.com/community/CompositeManager/ConfiguringCompiz
http://ubuntuforums.org/showthread.php?s=ffa26ed61380bc17b8164f969e1e317f&t=809695

Diagnose script.
http://forlong.blogage.de/entries/pages/Compiz-Check

Output of compiz --replace (immediately after initial login):

$ compiz --replace
Starting gtk-window-decorator

(screen flashes and I have desktop effects).

I ctrl-C and effects are off again (actually no window decorations at all). I then turn on desktop effects in the "Appearance preferences", and re-run compiz --replace. This time it doesn't say anything, and effects are running.

Here's the output of that diagnostic scripts:

Gathering information about your system...

 Distribution: Ubuntu 10.04
 Desktop environment: GNOME
 Graphics chip: nVidia Corporation NV44 [GeForce 6200 TurboCache(TM)] (rev a1)
 Driver in use: nvidia
 Rendering method: Nvidia

Checking if it's possible to run Compiz on your system...

 Checking for texture_from_pixmap... [ OK ]
 Checking for non power of two support... [ OK ]
 Checking for composite extension... [ OK ]
 Checking for FBConfig... [ OK ]
 Checking for hardware/setup problems... [ OK ]

It's as though my preferences aren't being saved, or when I log in it decides to ignore them?

Probably related: I have 4 virtual desktops. I just noticed that if I set them to two rows, when I log back in (and effects are also off) it always puts all four in a row again (so really seems like a preferences thing...)

Compositing of Metacity is disabled in gconf-editor?
/apps/metacity/general/compositing_manager = false

Verify backend and desktop integration in CCSM.
http://wiki.compiz-fusion.org/CCSM

Did you try to save current running apps?
system preferences -> startup apps

Checked that metacity compositing_manager is false.

Also confirm backend and desktop integration in CCSM.

Saving running apps in system preferences solved the problem! (that box was not previously checked) (both desktop effects and the virtual desktop layout issue I mentioned).

Thank you for the help!

> that box was not previously checked

Yep, it isn't default. Glad it helped.
Although I've never used 'safe running apps', I didn't experience your issue. But what I did through several upgrades/installations was only importing a backup profile, which avoids to verify the settings all over again.