The white 16GB model appears to be the one seeing the most demand, as it is unavailable in many stores.
Does this mean that AT&T and Apple officially underestimated the initial demand for the iPhone 3GS? Perhaps they did. It wouldn't be surprising that they expected fewer existing customers to pay the enormous upgrade fees ($200 more compared to new or qualified customers). Turns out, many did, as evidenced by the huge initial sales weekend, with over one million iPhones sold.
At the same time, Apple has re-launched the tool it used last summer that allows potential customers to check inventory levels of particular iPhone models. People interested in determining if the iPhone they want is in stock before making a trip to the Apple Store can use the tool on the Apple web site to take a peek at inventory levels.
Since the shortages are real, be sure to check online before hopping in the car.