Thanks all for the input. I upgraded from 1.06 to 1.08b5, following the advice of clearing the config before and after the upgrade. Mac OS 10.6.1 now works find, as does 10.5.8. Additionally, I can now connect my Ubuntu 8.10 and 9.04 boxes to secured shares (they used to fail just like os 10.6).
One quick comment about FW 1.08b5: When you back up a config file it is now in a binary format, whereas the 1.06 file was plain text. This was a bit of a pain rebuilt my new config and wanted to diff it against the old on to see if I got all the users, groups and shares correct. If I could have my way, I'd prefer text-based configs over binary. Just my $.02.