via ars technica (http://arstechnica.com/apple/news/2012/03/hands-on-and-questions-answered-third-gen-ipad-third-gen-apple-tv.ars):
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 the ability to output 1080p (the second-gen can play 1080p movies but only outputs 720p) 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.
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 "single-core version of the A5" and it's not the same as what's in the iPad. The CPU is capable of handling 1080p HD video. The spokesperson also claimed that users "only need an 8-10 megabit Internet connection" in order to take advantage of 1080p movies from iTunes, but speaking as someone with experience with the second-generation Apple TV at a mere 720p, this depends highly upon your individual ISP. (Insert grumbling about Comcast here.)
And despite rumors that Siri would come to the third-generation Apple TV (and continued insistence by some publications), there were no indications that Siri was anywhere on the new Apple TV interface. We are confident in saying that Siri is not currently available on the device, though that could certainly change with a software update.