ISA Server 2004 Service Pack 2 is the latest collection of updates, fixes and enhancements to ISA Server, for both Standard Edition and Enterprise Edition.
SP Numbering: There was no SP1 for Enterprise Edition, because it was roughly equivalent to Standard Edition SP1. Now, they both get an SP2 release.
What's New? - BITS caching for WindowsUpdate, WSUS and SMS, compression and caching support for forward and reverse proxy requests, and more (for a heavier but ultimately more satisfying read, see the SP2 Tech White Paper).
Thought o'the moment: when was the last time you backed up your ISA configuration? Just right-click at the top level and Backup, and put the XML file somewhere safe, while you think of it.