I received a question today on whether you can run Forefront for Exhchange on SBS 2003. Turns out it's tested:
“Whilst Microsoft haven’t yet made any public statements about the Forefront server Products – we can confirm that Microsoft Antigen 9.0 has been successfully tested on the SBS build of Exchange Server 2003. The SBS team is currently in the process of integrating the current Forefront for Exchange as a part of the next SBS release – but as yet, no dates are confirmed for this.
There are a couple of caveats to this solutions, firstly; installing Forefront for Exchange(FSE) and Forefront for SharePoint(FSSP) on the same box is not supported, currently only one of the Forefront products can be deployed onto a single system at any one time. Secondly; when Exchange and SharePoint are present on the same box, deploying FSSP is known to result in some issues and we are currently not recommending this configuration. These issues are to be resolved in the forthcoming release of the Forefront Server SP1 package”.