Once we make the build use xpidl.py for generating headers (bug 578478) and then typelibs (bug 578788), we can remove the binary xpidl completely. We can also remove our build-time dependencies on libIDL and glib, which is very nice.
We'll also have to tweak the SDK packaging to make it package xpidl.py in xpidl's place.
Kyle are you working on this? If not I'd be happy to steal it :)
Actually, I did this already in Bug 677922.