Apple generally begins to thin inventory prior to new product launches. So the growing shortage of iPhone supplies at Apple Stores could mean that the device is going to be refreshed.
Piper Jaffray analyst Gene Munster said that there is an "80% chance that a new version of the iPhone is coming earlier than anticipated." Others disagree.
Apparently AT&T stores in U.S. and retail outlets in other countries where the iPhone is available are not short on supply at all. On top of that, the FCC hasn't approved a new version of the iPhone. FCC approval is mandatory, and generally occurs up to two months before new phones are available for sale. It is notable, however, that occasionally the FCC approves phones immediately prior to launch.
If the existing product is to receive a minor memory refresh, no new FCC approval is needed. If it is to receive new radio capabilities -- such as support for 3G networks -- it would need new approval by the FCC.
Whichever angle you believe, something is going on. Exactly what that is, however, is anyone's guess right now.