Small Business Server is, by definition, also a Domain Controller. Since a Small Business Server can not have an additional Domain Controller added to it, you can't cluster the entire machine. I don't see any real issues with clustering the Exchange or SQL servers....

I don't believe that any testing has been done with SBS either.

Actually you can have additional domain controllers in a Small Business 2003 Server based Domain.

Pasted forom the Microsoft SBS 2003 FAQ:

Q. What number and types of servers can exist in the Windows Small Business Server 2003 domain?

A. There are no limits on the number or type of servers that can exist in a Windows Small Business Server 2003 domain, with the following exceptions:

• Only one computer in a domain can be running Windows Small Business Server 2003.

• Windows Small Business Server 2003 must be the root of the Active Directory forest.

• Windows Small Business Server 2003 cannot trust any other domains.

• A Windows Small Business Server 2003 domain cannot have any child domains.

• Each additional computer running Windows Server 2003 must have a Windows Small Business Server 2003 client access license (CAL).

• A Windows Small Business Server 2003 domain can have no more than 75 CALs. You can use CALs for each user or for each device.