Hmm... Ubuntu will probably run on ReiserFS. However, RFS is a bit more complicated than ext3. Some normal Ubuntu operations may require different set ups in order to work on RFS. For instance, I know from experience that hdparm will not work on RFS without the kernel modules loaded at startup. You'll need to know how to make these changes if you plan on using RFS. I have 7 distros on my system and only use RFS for one (SuSE).
I could be wrong about this, but I don't believe you can "convert" file systems once a distribution is installed. I believe you'll have to reinstall Ubuntu and format the partitions as ext3 in order to fix that.
Wait for some more experienced advice before jumping the gun, though.
Regards,
~Eric