If you mess up your GRUB configuration on an Intel Mac dual booting via rEFIt (or rEFInd), there are several things you can do.

There's all sorts hints and tips that may or may not work on your setup and are all very particular to your partitioning scheme, but I found a few to be quite informative, especially in understanding  the underlying technical problem (ie why GRUB is such a pain to get working right in these setups).

https://bbs.archlinux.org/viewtopic.php?id=99097
http://mac.linux.be/content/problems-refit-and-grub-after-installation
http://ubuntuforums.org/showthread.php?t=1704357

In the last one, the author of an important EFI / GPT-MBR hybrid utility chips in some advice and information. I would try to avoid doing a hard recreation of your hybrid MBR though. One thing you will definitely need is some type of live boot disk.

After trying several bits and pieces of the above advice, and apparently failing to properly reinstall GRUB, I went from a working GRUB that just couldn't find my OSes after selecting to them to a GRUB that never got past displaying "loading".  If you get that far, then you may have to force an install.

Based on this post, I wrote this script for my set up. And that fixed things.