ISA2004 SP1 as RTM'd and will be available to customers for download soon. Remember that you will need to configure your network manually as the networking wizards in SBS2003 RTM doesn't support ISA2004 on SBS. SBS will be supporting ISA2004 SP1 in our SP1 release.
One more blurp about ISA2004 SP1: VPN is now fully integrated with this version. Meaning ISA2004 on SBS will fully control RRAS settings. Any changes made through the RRAS snapin will be overwritten by ISA2004. Also dial-in only is not supported with ISA2004, which SBS is trying to fix right now. So wait for SBS2003 SP1.
Windows Server 2003 SP1 is in RC2 right now and some have installed it on their SBS machines. There are some issues in WS03 SP1 on SBS2003 RTM and those issues are addressed also in SBS2003 SP1. One issue that is common is that the Remote Access Wizard will fail after hanging for a long time.