Rugaby is right. Fall 2012 is the most likely release date. Apple has been doing this for a while and they've always stuck to annual releases. It's not outside the realm of possibility that it will be released in the spring, but it wouldn't be characteristic of Apple, and there haven't been any production rumors or parts purchases to indicate it would be any sooner than late summer or fall.
Why do people call the next iPhone the iPhone 5?
Some people refer to the upcoming sixth generation iPhone as the iPhone 5 because the rumors and predictions for the 4S were that it was going to be called the iPhone 5. When people found out the 4S was going to look exactly like the 4 and not have a larger screen, they assumed the next phone,
the real upgrade they were waiting for, would be called the 5. Many even thought it would be released sooner than a year after the 4S (like a month or two later).
It's unlikely that Apple would call the next iPhone the iPhone 5. It's illogical if you put any thought into it. The only reasons to ascribe a number to the phone are as follows:
1. If it corresponds with the generation (which would be 6).
2. If it corresponds with the network (which would be 3G or 4G).
3. If it looks like a previous phone that was named after a generation. They then add an extra letter to differentiate it from the previous phone. If the sixth phone looks like the 4 and 4S, it would be more likely that it would be called the 4X (or something like that) than the 5.