lammert - 1:14 am on Apr 13, 2010 (gmt 0)
This is a known bug in the Debian/Ubuntu installer. It sometimes doesn't recognize CD drives even though the Linux version will happily recognize the drive once installation succeeds. The solution depends on your hardware, but changing the drive settings in the BIOS might help. Some people had success by changing the SATA setting from AHCI to something else, and after installation back to the original value.
Changing drive settings in the BIOS is dangerous and you should always be prepared for a situation where you can't access the information on your hard drives anymore. Changing BIOS settings to install Ubuntu on a new drive is fine, but using it to try to install Ubuntu in a separate partition on a drive which has already a working operating system on it (like Windows or Linux 32 bit) is highly discouraged.