Running Mac OS and Ubuntu 10.10 booting with rEFIt then GRUB. Updated 10.10 to 11.04 and accidentally put GRUB on the wrong volume (I think swap somehow). Can't boot into Grub. Can I install it onto correct volume using Mac OS?
You should boot from an Ubuntu live-cd or something like supergrubdisk to install GRUB in the correct location.
For some more information, you could check out this wiki page which contains a few other possibilities and general useful information.