Exchange Server 2007 Service Pack 1 (SP1) shipped earlier today. It incorporates a myriad of enhancements, which arguably should have been in the RTM product. I'll be installing this at home as soon as I can! Be sure to check the system requirements first (an update to .NET Framework may be required for some installs).

Update: I've successfully deployed SP1 on my home Exchange 2007 server, and everything appears to be working fine (after the second attempt). The first attempt failed due to some Hub Transport role configuration issues (which in turn caused a bunch of things to fail until I could correct the issues and start SP1 setup again).