Exchange 2003 uses Active sync, 2007 & 2008 use Exchange Web Services (EWS).
Iphones use activesync and work well, Snow Leopard uses EWS.
I too was looking forward to getting SL for all our Mac users, so the whole company could finally use the same mail cals & contacts, but as usual, 'someone' messes it up for the majority.... why didnt they stick with active sync, as a LOT of companies, us included, are staying with 2003. It works, is stable, and does not require stupid money and hassle just to get the latest Microsoft offering. (I have stayed with XP companywide too)
The silly thing is, exchange 2007 & 2008 ALSO have active sync (as far as I have read) ?!? but EWS is supposed to be better.
SL will connect to EX2003, but only using IMAP, so pretty useless and no cals or contacts, only mailbox view. Dont waste your time trying to get SL's exchange connector to work on 2003, it wont.