Arduino pro mini: 5v vs. 3.3v

What exactly is the difference between the 5v and 3.3v versions of the Arduino Pro mini? Since the Pro Mini doesn't incorporate any voltage regulation circuitry (that I know of), and the Atmega chips usually run off of 2.7-5v, what's different between a Pro Mini 3.3v, and a 5v Pro Mini just hooked up to 3.3v?

Well there is a voltage regulator on board, either a 3.3vdc or 5vdc. The 3.3v version has a crystal frequency of 8mhz and the 5v version uses the standard 16mhz crystal. Here is the product description: