Binary package hint: ubiquity-frontend-kde
I used the Kubuntu 9.04 RC. See the attached screen shot please. After I select the keyboard layout, Ubiquity asks me if I want it to try to unmount some partitions. The options to answer to this question are either "Continue" or "Go Back". These answers are very confusing, I'd expect to answer with "Yes" or "No", or "Unmount" or "Don't Unmount". Besides that, the dialog mentions the same disk twice.
On a second try through Ubiquity to take a screen shot, I noticed that choosing "Continue" would make the same dialog always re-appear, the only possible choice was "Go Back" after which it ironically continued the installation process, to the "Prepare disk space" section.
I should probably mention what storage I had in use at that moment. I had a Live USB with a vfat (?) partition for Kubuntu, and separate ext3 partition for persistence. My normal hard drive had an ext3 partition for Ubuntu, a swap partition (don't know if that one was mounted, but didn't see it) and an ntfs partition for Windows Vista. I also had an external hard drive connected with eSATA formatted as ntfs. That's five partitions, but when I tried going through Ubiquity the second time to get a screen shot, some partitions were still unmounted after my first try with Ubiquity, that's why only sdb is seen in the screen shot.
The code is set up to ask "Yes" or "No" for this question, and "Go Back" and "Continue" for all other boolean questions.
I cannot reproduce your bug on the 20090419 daily-live CD. Can you please try the installation again in debug mode (`ubiquity -d` in a konsole window) and attach /var/log/syslog, /var/log/partman, and /var/log/
Thanks!