While surfing around for anyone that got Postfix 2.1 running under OS X tonight, I came across this article at AFP548.com. Not only does it give info on getting this running under Mac OS X 10.3, it also includes info on enabling PCRE, or Perl Compatible Regular Expression, a useful feature if you have more involved mail filtering rules set up. Most basic setups will use regex, more involved rules will require pcre, I've seen this most with some user created Spamassassin filters.
Also described are including support for SASL (Simple Authentication and Security Layer), but the article includes a (bad) link to an older version of this software, and isn't clear why the latest version (available when the article was published) wasn't used. I'm trying to contact the author for clarification. Also, the article seems geared towards OS X Server, but I'm assuming it should work well under standard OS X.