Actually the problem is a conflict between grub (used for CentOS) and grub2 (used for Ubuntu 10.x). This is what is likely causing your kerfuffle. Fixing it may be a problem and I avoid dual-booting my systems for just this sort of problem Instead, I have swappable system drives in a hot-plug carrier. I usually run other operating systems in a virtual machine, but for times when I need direct access to the system hardware, I just shut down the system, swap the system drive, and restart it.
Doh! Double posted this reply (more or less)... I thought I had hit the "cancel" button.