TheNextWeb now reports that that iOS 6 resolves this issue by forcing all devices to re-enter credentials if your Apple ID settings change.
According
to a source with knowledge of Apple’s steps to correct the matter, the
issue has been fixed in iOS 6 through a variety of checks placed on
iMessage. The most important of these is that Apple now uses its push
systems to force a user to re-enter a password to use iMessage once your
Apple ID credentials have been changed.
Now, if your iPhone is
stolen, simply changing the password of your Apple ID will force the
stolen device to prompt for a password in order to continue receiving
and sending iMessages from your account. In addition, wiping your
iPhone remotely using Find My iPhone will now disable the stolen device
from receiving iMessages.