I don't think it's reasonable to expect Google Talk to remain indefinitely though - if you need XMPP you at least want an exit strategy if Hangouts isn't suitable. It's quite telling that the Hangouts app on Android is called "Hangouts (replaces Talk)" and takes Talk's URL in the play store.
Despite the clear warning signs, I am willing to put money on people whinging in a year or two when they shut down those servers, just like everyone got all upset about Reader.
Will GTalk be able to communicate to users of Hangouts, though, or will everyone be forced over to Hangouts in order to continue to speak to the people who move over? I'm guessing the latter.
Yes, for now they're basically the same service. I'd imagine that this will change as Hangouts diverges more and more from what Talk supports, to the point where Talk will be killed due to disuse.
don't expect that to last too long. the google talk android app updates to become hangouts, and the chrome extension became hangouts automatically today.