I just tried that. This is what I did:
- I Installed refit (http://refit.sourceforge.net/). This step might not be necesary, but I installed it because I was worried about not being able to boot Ubuntu after the process.
- Then I installed OSX, leaving a free space on the partitions.
- I booted Ubuntu with a USB memory. To be able to do this, you might need to keep the option (left Alt) key pressed when booting. If you didn't install refit, use the command (Super) key.
From that, you just make sure that Ubuntu uses the free space and not the hole disk.
What problems did I find?
Well... it starts fine, but after 5 minutes my desktop freezes...
If you manage to install the video drivers correctly, let me know. I'm going to try with 12.04 and see what happens.