The man page for BridgeSupport claims that it describes API symbols which cannot be introspected at runtime. It also says, "Only classes where additional metadata is needed are described."
It seems the additional metadata is what kind of sentinel is used for variable-argument methods and which classes are toll-free bridged to which other types.
Also, no bridgesupport files appear for the iOS or iOS simulator platforms, though there is a tool which might generate this information based on headers.
IMHO, this does not solve the method dispatch problem, but it can add a lot of value for Mac OS X.
For my purposes - which is iOS development - I want to be able to call third-party class methods and even undocumented methods (in the simulator), so I'd like to keep the dynamic dispatch.
As an optimization on iOS, it would be cool to run some stats on the most commonly called function signatures and fast path those.
-Jason