I noticed my Nokia Lumia 920 with Windows 8 gets VERY hot sometimes. It becomes so hot on the backside, that I need to turn it off. It does not seem to be a deal breaker. In other words, it does not stop the smart phone's functionality. It is the same as with many laptops, Windows or Macs. After much use, they sometimes can actually heat up noticeably on the bottom. This is especially true when dealing with heavy video editing or other heavy duty multimedia usage or work.
My Windows 8 Lumia 920 last night became extremely hot after using the Nokia Maps version 3.0. The good news is that the Nokia Maps app is awesome. It not only shows directions, it actually vocally points you in the right direction via voice. I love it. And it is free. I just wish it did not get so hot!