Service Pack 2 for Windows SharePoint Services 2003 has been released last week on September 27.

You can download the SP2 here .
You can
download the full version of Windows SharePoint Services with SP2 integrated (slipstream) from here .

From my perspective, most interesting features added by this service pack are:
 - Whidbey (.Net Framework 2.0) support: both side-by-side with .Net Framework 1.1 and also support for running against Whidbey
 - Support for 64 bit - WSS with SP2 can run in 32-bit emulation mode on a 64-bit machine

Information for BizTalk 2006 Beta 1 Users
WSS adapter and BAS included with BizTalk Beta 1 require the preview build of WSS SP2 which has been made available through the BizTalk 2006 Beta program. 
The RTM build of WSS SP2 seems to not work with BizTalk 2006 Beta 1 due to compatibility issues (they are compatible with different builds of .Net Framework 2.0). 
BizTalk Beta 1 requires .Net Framework 2.0 Beta 2. WSS SP2 preview build is compatible with Beta 2 build of the .Net Framework where the RTM build of WSS SP2 seems that it's not or at least I wasn't able to get it to work.

BizTalk 2006 Beta 2, probably shipping sometime in November 2005, will most likely work with the RTM bits of WSS SP2 and .Net Framework 2.0.

Information for BAS feature users in BizTalk 2004
If you have installed WSS SP2 build on a SharePoint machine with BizTalk 2004 and BAS feature then you probably have to apply a BizTalk 2004 hotfix, otherwise the SharePoint messaging feature in BAS will be partially broken. For more details see KB article http://support.microsoft.com/kb/908359