John, here is a guide on how to install Exchange SP2 on SBS2003:
Unfortunately, you will not be able to play with mobility features until your carrier/vendor releases the Mobility Security & Feature pack.
Can I install Exchange Server 2003 on a Hyper V guest running Windows Server 2003 at this time?
Saif - As for "can you", the answer is probably yes. But is it supported? No.
See this page for details of support for Exchange under Hyper-V. It has to be Exchange 2007. http://msexchangeteam.com/archive/2008/08/19/449621.aspx