I would recommend asking in an SBS related newsgroup--SBS has an excellent
and high-energy group of MVP's, and not only will they have a strong opinion
on this, but they may be a useful source of information on other SBS issues
you may have.
Here's a web reference to one SBS specific group--there are others,
including ones specific to SBS 2003, I believe, but the web interface makes
them harder to find, I'm afraid.
http://support.microsoft.com/newsgroups/?pr=1167
However, I can say that I am running Microsoft Antispyware on two SBS-2000
servers with no ill effects.
Earlier builds had a defect which could result in a log file reaching a 2
gigabyte size before it was limited by the NTFS file system. The current
build doe not allow that to happen. That was the only issue I had with this
product on the server. It also may throw out some spurious error messages
when you have Administrative Terminal Services sessions running, but it is
still useful--both daily scans and real-time protection work.