Can do DNS, but nothing else, over WPA

I can no longer access the Internet over WPA from my Toshiba Satellite running stock Ubuntu 5.10 with wpa_supplicant 0.4.5 using the ipw driver. It worked three days ago, and now it doesn't. WPA still works on a separate machine running OS X.

/etc/network/interfaces ends with:
auto eth0
iface eth0 inet dhcp
pre-up /etc/init.d/wpasupplicant start
wireless-essid London Street
wireless-key s:xxxxxxxxxx

/etc/wpa_supplicant.conf contains:
network={
    ssid="London Street"
    key_mgmt=WPA-PSK
    psk="xxxxxxxxxxx"
    priority=5
}

I can nslookup domains, but i can't ping them.