Assuming you're using db files for Bayes, rather than SQL (as suggested in another thread), mail delivery is happening on a different group of machines, which is still running an older release of Debian - so the db libraries are likely different.
It'll be even more confusing when DH starts upgrading mail machines, as different mail machines may be different Debian releases, for at least a short amount of time.
w/r/t version, you'd see 3.0.x on the new (Sarge) user machines, but this will never be used. 2.20 is on the old (Woody) mail servers. The Junk filter machines were running different versions last I checked (3.0.4 backport on one and 2.64 or something on the other) - however that may not be the case currently (you could look at the headers of junk-filtered messages to determine this). In any event, you shouldn't use or rely on the installed SpamAssassin binaries on the DH servers, as they may be inconsistent with what's on the mail machine, and will likely stay the same across a release of Debian (meaning it will get old and moldy really quick).
If you can't or won't use the junk filter, do your own install - there are some good howtos out there. The bayes stuff should work out better after the mail servers are also upgraded. You could try testing the install, but then hosing the bayes db entirely and see if it works on the mail servers then - or you could install your own libdb in your home dir and compile SpamAssassin against that - that would be a big PITA, though.
Also keep in mind that the version of Net::DNS on the mail servers is probably old.
this post (and check out the rest of that thread).
This post says a lot of the same stuff.