There have been a few people messing up the password during the setup of Ubuntu on the PS3 so heres a quick guide of how to reset it. You will need to know what your ubuntu username is to set the password for it.

At the kboot prompt

kboot:

type: sudo -i

then type: write mount -o remount,rw /

then type: passwd (username)

then write password twice

type: exit

then hit enter

This will now boot to the Ubuntu logon screen and you can use the password you just set