Yep, Internet Security and Acceleration Server 2004 information is now becoming available on Microsoft.com.
ISA 2004 incorporates much of the feedback that the ISA community had about ISA 2000, and - so far for me at least - is just as reliable and ten times more configurable. What's new? Lots.
A word of warning - it's different enough from ISA 2000 that you'll probably want to skim the documentation and have a play with it before leaping in: you can block/allow traffic in any direction, NAT networks, route networks, set up internal and external listeners on networks, filter secureNAT web publishing requests... (the list goes on).
The default configuration is quite heavily locked down. If you're planning on experimenting with ISA on a Domain Controller (or any other production server that clients access, for that matter) keep in mind that you'll need to create access rules between the internal network and ISA Server (unlike ISA 2000 where the internal network was trusted). To allow certain infrastructure protocols (DNS, DHCP) to ISA, you can edit the System Policy, and create additional access rules.
Technet information (mostly geared towards planning, because we're at that point in the lifecycle) : http://www.microsoft.com/technet/prodtechnol/isa/default.mspx
RTM version is also available through MSDN Subscriber Downloads.
Trial version: http://www.microsoft.com/isaserver/evaluation/trial/
ISA Server Community on Microsoft.com
http://www.isaserver.org/ Tom Shinder's excellent ISA resource.
Got ISA 2004? Got Comments?