Boy it's tough to predict the future. Toyota has historically limited production to maintain a 2-3 month backlog on orders, but the variation has been pretty large. A surge in gasoline prices that lasts more than a month would probably make the backlog grow huge. If the upcoming other makes/models of hybrid really do show up, the backlog could shrink, possibly even resulting in immediate showroom availability of Prius (depending on the relative desirability of the new hybrids). A reasonable strategy would be to monitor the forums/boards frequently, starting early in September, to get a feel for the backlog. Then put in your order, timed with that information, to arrive in January. It sounds like you can tolerate a month or so variation, so that should work out reasonably well. If in September the apparent backlog is more than 4 months, you probably weren't going to get the timing anywhere near right anyway. If there doesn't appear to be much/any backlog, show up at the dealer in late December hoping for a great year end deal. If none is available, buy at normal prices in January.
Good luck with your car acquiring plans.