I wouldn't worry about it. My ass was waiting in line on June 29, 2007 in Chico, California last summer in 95 degree heat and than I flew down to Ft. Lauderdale a couple days later and there were still tons of iPhones down there at the Apple store the Monday after the launch.
Plus, most of the "normal" public will be put off by the $30-35 dollar data plan that comes with the 3g iPhone. In this economy, ATT is making a big gamble that people would pay this much for a cell phone/data.....
Remember even though the $30 "iphone" data plan is the same price ATT charges for it's BB/Windows Mobile phones, these only make up less than 10% of ATT's regular customers. Will/Can ATT convince the average public to folk over that kinda of money for data. Cheap regular high speed home internet from Cox Cable at 768kps only costs $16.99 (in Fairfax County Virginia) so paying $30 is a lot of money in most people's mind.
Thus, I do not think the 3g iPhone will be hard to get.