Well you could do this (though it's kind of a workaround):
Oh, and I should add before you start doing it, that I don't have MobileMe, so I can't promise no complications will arise there.
Open Address Book and go to File > Export > Address Book Archive...
Save your Address Book file somewhere, and give it a name to specify iPhone or Blackberry (I'll just say you saved it as your iPhone address book from here on out), or however you wanna name it. Save it wherever makes sense for you.
Now go back to Address Book, and set up your AB for your Blackberry. Do the same as above, but name it for your Blackberry. You now have two separate Address Book archives stored.
Whenever you want to switch which one you're working with, go to File > Import > Address Book Archive... and load it up. But be sure to export the one you're currently in first, cuz the changes aren't saved to the archive. You probably just want to overwrite the previous version.
Then, whenever you want to sync your Blackberry, load up the Blackberry AB. Whenever you want to sync up your iPhone, load the iPhone AB. Again, I can't promise this won't get sticky with MobileMe. My best guess would be to be careful only to sync with MobileMe when you have the iPhone AB loaded.