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.

This is likely a conflict between grub (CentOS) and grub2 (Ubuntu) boot loaders. This is one of the reasons I never dual-boot a system, but either run other operating systems in a virtual machine, or if I need direct access to the hardware, I have hot-swap system drives that I can swap out as necessary. In fact, I have three such. For about $75 each for a 320GB system drive and carrier, the price is decent. My biggest problem is finding the key... :-)