Attempting to link against the libmongoclient.a provided in the 1.2.0 and 1.2.1 release tarballs yield unresolved externals when using a different version of boost. This is apparently expected, and I was encouraged to compile it myself.
The crux of the matter is that compiling the client lib shouldn't require spidermonkey. A simple hacky change to the SConstruct file:
compiles fine and links into a program fine. Its obviously not the right fix, but it does show that it isn't actually used in the client code.
In addition, perhaps libmongoclinet.a shouldn't be distributed in the tarballs, or it should at least specify which version of boost it was compiled against.