Say hello to Lumia 810 for T-Mobile USA, phone that is variant of Lumia 820. These phones are indeed almost identical, but not totally so. First thing you spot is the slight differences on design where Lumia 810 takes more blocky look over the more round Lumia 820. Both feature interchangeable covers.
When it comes to size there’s very little difference between the two. Lumia 810 is bit lighter than Lumia 820 though.
Battery life on paper at least shows quite big of a different on 3G talk time between these two phones. Lumia 810 getting on with 10.2h talk time (3G) and Lumia 820 with 7.5h talk time (3G). Nokia hasn’t yet provided information about the battery size on Lumia 810.