According to the Wall Street Journal, Apple is already 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 colours, 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 autumn with the 4S in 2011. [WSJ]