2nd Generation Apple TV to get UI/1080p update? Now that the Apple TV has been announced and is available, will Apple be releasing a software update for the 2nd generation Apple TVs? I'd like to know if the new UI will be supported on my ATV and whether or not 1080p support will be made available as a software update or if it is dependent on new hardware/processing.
Pretty sure that there are hardware differences allowing support for 1080p, not present in prior gen Apple TVs. So if that is the case, then a software update can't add in that support. We will not know for sure until we can see the spec and a complete teardown of the new Apple TV. We might be in luck for a UI update, though.
So according to ars techinca's Hands-on and questions answered: third-gen iPad, third-gen Apple TV

Apple TV
First let's start with the Apple TV. The device looks the same as the second-generation device and, in fact, the new interface that Apple touted during the event will also be coming to the second-generation version via software update. The only thing new about the third-gen Apple TV is 1080p capabilities plus a more direct link into iCloud for photo syncing. If all you want is the new and improved interface (which I do like, as an Apple TV fan), you'll be able to get it from Apple on your second-gen device without having to pay anything.

And as for hardware changes that's a yes to according to the same article as well.

We asked whether the third-gen Apple TV had the same A5X processor as that in the new iPad (as was previously rumored), but an Apple spokesperson told us that it has a &quot;single-core version of the A5&quot; and it's not the same as what's in the iPad. The CPU is capable of handling 1080p HD video.