Wow, I've had an overwelming comments or emails from my Vista Update for SBS 2003 about when the ISA 2004 Firewall client will release.
Asking around internally, you guys are in luck! The Vista Firewall Client is targetted to ship around the same time as the consumer Vista's and the Small Business Server Vista Update. So, come January sometime, you'll have all three in your happy little hands. :)
Hope this helps with your planning, now you can at least target a date!
Oh, and just for completeness, here is the Updated Firewall client for 32 and 64 bit hardware running on Windows XP.