The best thing about active sync compared to BES is that your email does not get routed through the RIM servers in Canada. We've had issues where the telco had issues routing to Canada for days at a time while our Internet access was fine. Active sync is direcetly back to your server.
SBS 2003 if configured in a standard way will support active sync already, I think you only need SP2 for exchange 2003 for this to work.
The only other thing I'd consider is getting trusted certificate so that you don't have that hurdle with every mobile device. You can get a free certificate (1 year) from startssl.com
