I've just done exactly what you've been doing but with Centos 5. (100% Red Hat Code apparently)
Because I loaded Ubuntu second I've got Grub2 installed which doesn't appear to like Centos. This may be because Centos has arranged itsself in two partitions; /boot and a LVM partition. The latter may be due to the fact Centos is installed on a three disk hardware RAID 5 (HP Smartarray 600, I think)
I managed to fix it last night by running up Ubuntu and editing the Grub 40-Custom file which is read when
is run. In /etc/grub.d I think, check the Ubuntu page mentioned below.
I mounted the Centos /boot "disk" and checked the entry in /boot/grub/grub.cfg (that's the one on the /boot "disk", not the Ubuntu /boot!) . Copied the Centos entry and pasted it into the 40-Custom file which I edited using nano.
You have to modify the entry as Centos uses Grub1 and the entry is a bit different. I googled "grub2 Ubuntu" and came up with a really good Ubuntu howto page. It'll also explain where the files; 40_custom, etc, live.
There's one line I added to the 40_custom file that mentions a floppy and a disk UUID... I used GParted to find the UUID No to allow me to insert this line and change the UUID. Note the pasted in line which is changed from "kernel" to "Linux" has a /dev/sdxn pointer rather han a UUID like the Ubuntu entry... Leave it with the /dev... entry.
I then ran
which found my other OS'; Ubuntu 10.10, Ubuntu 10.10 Server, Fedora 15 and NO Centos
This is because you're manually entering it!
Rebooted the system and picked Centos from the Grub menu... Tah Dah!
All appeared OK.
If you can make any sense of my inane ramblings, I hope you get your system to work!