I did know that ACT! isn’t supported on SBS 2008 as per the System Requirements, but didn’t know exactly why it can’t be till I saw this post about a Microsoft bug: http://forums.iis.net/p/1151288/1877461.aspx.
ACT! Premium for Web 2009 uses IIS6 compatibility features and on 64bit, this requires the 32bit functionality enabled which is broken in SBS 2008 as per the post above.
In addition, I’ve been told not to remove RPC/HTTP as suggested in the article as this appears to cause a cascading failure. RPC/HTTP will not function even when re-installed.
The current word from Sage is “ACT! Premium for Web 2009 was / is not supported on this operating system as it wasn't released until November 2008 whereas APFW2009 came out in August. There are no current plans to support this.” Although, based on past experience, if MS fix their bug, it’s likely Sage will QA test this and hopefully certify it for ACT! 2010 (12).
I have heard of users with ACT! Premium 2009 (non web) who have run without issues on SBS 2008. So, if you need to, you might try it… but remember, it’s not officially supported and any issues may be difficult or impossible to resolve.
On a more general note, I typically find SBS to be a problem. Running a Domain Controller, Exchange Server and web server on the same server is a security and resource disaster waiting to happen and should be avoided if at all possible.