It depends on the amount of airflow really. Basic physics tells us that hot air rises, so the bulk of the heat from the TV will go up not out. So if you mount your Xbox, you'll want to make sure that there are upward facing ventilation holes.

If you can keep a couple of inches between the TV and Xbox, I think you'd be fine. I'd also mount the Xbox as close to the bottom of the TV as you can. I do also suggest you consider how having the Xbox behind the TV will work when it comes to loading/changing game discs. Sometimes it's those little details, which are so easy to overlook, that end up killing the entire idea.

You also need to consider if there will be any other devices behind the TV, such as a surround receiver. Those tend to put out a lot of heat as well, and would help to saturate the air behind the TV which is what will carry the heat away from the devices. So, if it's JUST the Xbox, and maybe something simple like a DVD player, which will never be on at the same time as the Xbox, you're probably fine so long as you can give it 2-3" of space on each side. Anything else, and I'd suggest a rethink.