I write the solution for a problem that USB drive cannot be mounted automatically on Ubuntu 9.10.
Firstly, install gnome-mount.
sudo apt-get install gnome-mount
Secondly, go to System > Preferences > Startup Applications. And add this command.
gnome-mount -p xxx (change xxx to the volume label)
Finally, log out the system and it should work then. However, the problem still occurs after unmount and unplug the USB drive. To fix it, log out the system and then plug in the USB drive again. It is not a very good solution by the way. 🙁
Also, find the bug report here.