That's a possibility, but I don't think it's in Google's general nature to do that: they were probably more concerned about closing the blatant console shell bug than they are about keeping us from getting root.
In addition, if they pushed a test-key update, I think that would have to go to every G1 phone, and that may cause user inconvenience with phones that are using the normal release keys (ie, the phone restarting and failing on an update due to a bad key)... I guess we'll see.
I suppose that the real solution would be to sign with a legitimate private key, but that opens another can of worms: Who holds the key? What happens if they disappear off the face of the earth? Etc.