I hate be that guy, but it appears I have to be. Well, you might just be unlucky, but anything wierd about how you store them or things the kids might do when you are not looking?
Where I live is really dusty, so I have to clean my computer and Xbox every so often. Do you make sure it gets enough venting? I have a friend that even has his Xbox and PS3 Slim on the carpet and they are ok so far (his PS3 Fat died the same week the slim came out - timing!)
Anyway, sometimes things just happen. On my slim I had the hard drive getting noise, and I notice it was just a little bit off in alignment. Unplugged it, adjust the tray back just a tiny bit, and now it's fine.
Have they done an internal improvement to the Slim yet? It came out 2.5 years ago, they tweaked the older x360 all the time it seemed.
PS Hope it returns quickly. Do you only send in the console and keep the Hard Drive?