I had a triple boot with rEFIt OSX 10.6.3+W7 (boot camp)+Ubuntu 11.10 and everything was working fine.
Then I updated Ubuntu but when I tried to reboot I found only a blinking question mark.
I couldn't reboot from OSX system DVD (both grey and white disks)
I took off the HDD and connected it to an iMac with USB box: I couldn't read anything because the HDD resulted unformatted
I suppose that Ubuntu broke GPT.