Hi,
All the NAP enforcement methods require at least one Server 2008 machine running NPS as a NAP health policy server. For DHCP and IPsec enforcement, the NAP enforcement server must be running Server 2008. The NAP health policy server and the enforcement server can be the same machine, or different machines. If they are different machines, then the DHCP server and HRA server must run NPS proxy and forward authentication attempts to the NAP health policy server.
You can use SBS 2003 for the domain controller and other services, but you are correct that it cannot be used as the DHCP server for the DHCP enforcement method.
-Greg