Yep looks like something is up with GTalk/AppEngine.
I have posted something about it at Google and will wait to see if the problem resolves itself.
If not I guess I could stop checking for presence information and rely on delivery errors to perform a fallback, that may or may not get over the problem, but would require some 'ugly' changes to the code which I really would not like to perform.
ps: By the way GTalk notifications is down, but sending SMS via GTalk is still working.
(ie you can still send SMS via GTalk, but not receive GTalk notifications when you receive an SMS, instead for most people it will fallback to sending an email notification)