There’s always questions about virtualising SQL and Exchange servers. We know that Hyper-V performs really well and the advice is that you should be concerned about virtualising something that needs too much CPU or disk I/O. They’d be candidates for physical deployments. Everyone’s idea of large and heavy is relative. You might consider your SBS server to be heavy weight. You might consider a 4 server Exchange 2007 server deployment to be heavy. MS thinks of that as small!
The Exchange team has done a good post on virtualising Exchange 2007 and even give some sample architectures. They give 3 interesting solutions too, from the “small” deployment, virtual DR and an office in a can.