There is no reason at all the current AppleTV hardware couldn't handle rentals for example if thats whats coming. I wouldn't be surprised at all to see an AppleTV firmware update, considering its currently based on 10.4.7, would be a great opportunity to move it up to being leopard 10.5 based.
Whether Apple provide the previous gen (if its replaced) with the ability is another question
I can imagine a firmware update for rentals, but certain other things might require newer hardware, and nothing can be done about that except to upgrade.
And just about any technology company when the 2nd generation hardware comes out. Why do people think this is an Apple-exclusive thing???
Any of the likely updates coming will be handled via a software update. There is no reason for any new AppleTV hardware, except maybe hard drive size increases.
I think Apple (& the studio's) would be stupid leaving the current TV owners behind, considering it's an established user-base, ready and waiting for such a service. We'll see I guess.
Personally I wouldn't mind paying for an update, if it were worth while - having to replace hardware, just for rentals would be ridiculous.