I confirm this bug in Ubuntu 8.04 beta 1 LiveCD
I also receive this error: "Unable to scan drive for media changes. Cannot invoke CheckForMedia on HAL (...) "
I also test:
sudo mount /dev/fd0 /mnt/
and It works good - I can read from floppy.
Early 8.04 alfa works good, but few last alfas 8.04 have errors with floppy.