Android 2.2 (Codename: Frozen Yoghurt) brings a new feature which allows moving apps to the SD card. Unfortunately this only works for apps which bring a flag to allow this, i.e. not for the current versions.
To enable this globally, activate USB debugging and plug the phone to your computer. Make sure,
you have the Android SDK installed and
cd'ed to the
sudo ./adb shell pm setInstallLocation 2
Valid values are:
0--- Automatic (app decides, default)
1--- internal memory preferred
2--- storage card preferred
(seen in a comment on androidguys.com)
If you get insufficient permissions errors, run
./adb kill-server and try the above command again.