Hi Nyll.
My error also says that I am running (or trying to...) a BETA version.
The version I downloaded is 3.4.1.266581 which, as far as i can see, is not a Beta version.
(https://secondlife.com/support/downloads/?lang=en-US)
I also tried with a previous, non-beta version, just to check that and the error comes back the same.
Any advice anyone can give will be welcomed, though it would appear to me, that Linden Labs need to get on to this and write whatever drivers are needed for this. Ubuntu 12.10 has been around for a while now and it seems odd that there is little activity in keeping the viewer on track.
Regards,
SM