That is a bad idea actually. DHCP should be hosted on Windows otherwise things like registered DNS and the like *may fail.*
I also have noticed that certain routers don't handle redundant DHCP as well as they should.
I suggested that as an alternative for when the SBS server is down for maintenance. Since Exchange, file shares, etc., would all be on the SBS box, the only thing the secondary server would be handling in the case you mentioned is DNS.
