As mentioned, SBS is built on Server08R2, so it's otherwise perfectly compatible running as a VM. SBS is simply server 08r2 with a ton of stuff crammed into it (Exchange, Sharepoint, SQL, AD, etc), intended for small businesses that want all the good stuff but dont want/need 3-6 servers .
Although there's some people out there who think 'why bother' virtualizing a single server, in my view, you would need a pretty damn good reason not to virtualize a single server. Why would you choose to not reap all of the benefits of free virtualization? Add in a basic backup program like Veeam, and you have a server that's light-years ahead of physical when it comes to resilliancy, reliability, and expandability. SMB's could only dream of this stuff a few years ago. I've lost many hours of sleep recovering SBS 2000-2003 boxes back in the days of physical. Do you really want to go through that, when you can recover a VM from a backup in about 10 minutes?