A couple of SP1 KBs I had been waiting on went public while I was out last week. You'll definitely want to have a look at these ones:
KB.867633 - Intelligent Message Filter (IMF) Release Notes - This is the updated version of the release notes that answers all of the Frequently Asked Questions about IMF -- where to install it, why it's not working like you expect, etc.
KB.842106 - Mailbox remains in the original site after you move the mailbox to a mixed site in Exchange 2003 - This is not the title I would have given this one... might be more appropriately called something like “if you don't have proper two-way connection agreements for all sites and you move mailboxes cross-site with the new SP1 functionality, the stub mailboxes in the source site may not get cleaned up properly”. The workaround is to use the ADCTools to create proper connection agreements, but if you've been reading my blog (or the prescriptive guidance for Exchange 2003) you probably already knew that!
KB.867626 - New Error Correcting Code is included in Exchange 2003 SP1 - SP1 adds to the existing ability to detect checksum errors in the Exchange database some additional functionality to correct the error in certain cases! See the article for substantially more detail.
I'll try to post a few more as they are released in the next few days.