The fact that this thread is almost 2 years old is a little disheartening. I just recently switched to an iPhone 5 from a windows phone. Overall I'm pleased (I've been using an iPad for about a year), but I am disappointed that my iMessaging occasionally locks into portrait mode. A reboot is the only way to fix this. For the money a new iPhone costs and the duration of this issue, you would think Apple would have it corrected. Are there others who still have this problem, or is is somewhat of a fluke thing?
(FYI, the orientation is NOT locked, landscape mode works great in Safari, Calculator, and every other landscape enabled app. It's just in iMessage that it occasionally has issues.)