A good time for it to come will be with the iPad 3. This is because:
i) the iPad 3 using the A6 processor will free up supply of the A5 to allow its use in a new ATV.
ii) Apple are pushing mirroring quite a lot and the ATV is key to this (and actually marketed as and iPad accessory)
iii) The iPad 3 (along with iPhone 4S) will probably be able to shoot 1080p video - so an ATV supporting 1080p makes sense
It wouldn't surprise me if by the end of this year all Apple devices have moved to 1080p (and possible the introduction of a new codec next year to cut file sizes in half for the same video quality)