I see that you have resolved the issue by installing samba3, but I would like to post the simplest answer to your problem, without removing samba2.
the trick is in te samba.conf file:

For winXP and OS X until 10.5.x:
encrypt passwords = can be anything between yes or no

For Snow Leopard:
"encrypt passwords = yes" - mandatory, or you may have to change your OS x settings..

Best of luck,
Ed