I see vendors selling refurbished pSeries machines
with RHEL-AS or SLES ... so there is no reason Ubuntu won't run it. Check the CPU though, don't these use powerpcs?
The standard ubuntu is for intel cpus. But see:
... 8.04 has a ppc release.
No special license is required for the free software parts. Proprietary software may need a separate licence
Note - 9.04 is current, and also has a ppc disk.
Perhaps you want the LTR edition for long-term support?
If you install 8.04, you will also have a very large initial upgrade just to get all the security patches. If this is not an issue, go right ahead. Otherwise, the next LTR is not long away, it may be more judicious to install 9.04 and keep up to date, then stabilise on the next LTR release.