I've pushed patches to do this, and also added the full GHC version and arch (i386/x86_64) to the framework version number.
One annoyance is that if you have 7.1.20101023-i386 installed, then 7.1.20101022-i386 won't actually install any files (Console's log gave something to the effect of "newer version already installed" as the reason). I'm not sure if that can be fixed by fiddling with CURRENT_PROJECT_VERSION, or by fixing/removing the version number in GHC-relocatable.pmdoc/01ghc.xml or GHC-system.pmdoc/01ghc.xml.
I couldn't find any documentation for how OS X determines a newer version, or even where it gets the version from, since there are many many things scattered around that look like versions. There are also a few versions in Info.plist: CFBundleShortVersionString and CFBundleVersion. You can install with the cmdline installer, which will print those errors to stdout, but the GUI version annoyingly won't tell you if it aborted because it thinks you have a newer version. I'll download the latest build and mess around with it some more to try to figure out what makes version upgrades happen. The main thing to verify is that release versions with different patchlevels upgrade properly, since this is certainly something people will be doing.