'nuff said! See http://www.microsoft.com/isaserver/ for more information.
March 1 is the general availability date, but there's a trial CD available for order now. The trial download's currently scheduled for March 1.
I'm still learning about the cool new stuff in Enterprise Edition, but if there's something you're specifically interested in, please feel free to ask!
[Update] OK, so the question was asked about what was actually new and improved in ISA 2004 (next time, I'll just say "ask me in a week").
What's New And Different In Internet Security and Acceleration Server 2004 Enterprise Edition? And what does it mean?
More as I learn about it...