According to the Wall Street Journal, Apple is already getting started on building "a refreshed iPhone similar in size and shape to its current one" this quarter, and it might be out as early as the summer.
The report also says a "cheaper" iPhone, maybe in different colors, is still coming, but not at the same time as the new main release. It could be as soon as the second half of this year, apparently, but nothing specific.
A summer release would get the iPhone back on its original scheduling, which was originally pushed back to the fall with the 4S in 2011. It would also go a ways toward boning over anyone who picked up a T-Mobile iPhone 5 once it became available.
The most interesting thing about the iPhone back in the summer, though, is a precedent of accelerated release cycles. The iPad got an unexpected bump last fall, with a new processor, and this would be the second consecutive chip pushed out ahead of the typical schedule. Apple is freshly and heavily invested in designing its own chips, with its own custom cores, so a faster cycle could mean things are picking up in that department. [WSJ]