As stated in a Wired.com article, some users are reporting that their phones are overheating. So far there have been few claims, but since we all know Apple sold a lot(about a million) of these in the first week, there are bound to be more phones with the same issue. Apparently its due to faulty battery cells, and it is very possible that there could be thousands of affected phones out there. You guessed it, if your phone is affected you cant just swap out the battery for a new one, you have to either send it in for repair or swap it out at your local phone dealership(and we all know how that goes sometimes)!
I am still waiting patiently for my Blackberry 8900, no touchscreen for me!