I don’t think bbPress XML-RPC relies on PHP having an XML-RPC extension turned on.
Other peoples problems have been related to their hosting company not allowing remote calls to be made to the same server.
When this is the case the host generally recommends running the sites on two separate servers somehow.
There is some craziness in the way the forums component is architected to integrate via XML-RPC, but it got the component over the line for 1.0. Better solutions should probably be sought for the next release.
It’s my intention to make bbPress integration with WPMU and BuddyPress even simpler and better over time. Many changes have already been made to both WPMU and bbPress core to help BuddyPress along and that cooperation will exist in the future.