When setting up conditional forwarders in Server 2003, there is an option which says "Do not use recursion for this domain". This option acts only on the zone currently selected in the forwarders list. In the server 2008/Windows 7 snap-in, this interface has been replaced, and I cannot find an equivalent option. The closest option works at the server level, which I do not want.
Seems like you can still do it from command line, with /slave switch: