Being constructed out of aluminum, the more you use it for extended periods of time or have you brightness at higher settings, the more often and much quicker it will heat up. That being said, if your just snapping a few quick pics, or texting a buddy for a few minutes without putting your phone back to sleep and it feels actually hot not just warm, then you might have an issue. It really just depends whether you can live with it or if it's going to be something that will bug you every time you notice it. If the latter of the two is the case then I'd suggest exchanging for a new unit.
Hope this helps^^^^^^