I think the problem is there are a number of reasonable dates on which Apple could update. We can try to analyze Apple past actions to justify our speculations, but the truth is that no one really knows. In fact, I would think that it's entirely plausible that Apple has more than one date in mind; they could be monitoring market trends as well as how well their own product is selling in comparison to comparable models of different brands.
In the end Apple will do what is best for Apple. In any case these seem to be the most probable dates.
Late July - Mid August: Allows for iPhone hype to fade as well as time for Apple to gauge their own sales against upgraded competitors. If Apple suffers or sees a high demand for the new Montevina upgrade, then they will update to remain competitive. Plus, it is not unreasonable for Apple to release a newly redesigned and upgraded product during a promotion as they did with the new iMacs, though past action does not necessarily dictate what future action will be.
Mid September - It would be a logical move. Even though the student promotional sale is directed towards more "consumer" buyers (meaning those that will buy Macbooks/iMacs), there are students and faculty who are buying Macbook Pros. This will allow Apple to clear their current inventory and make room for the new model. As a business owner, there is no sense in keeping old products that will simply sit around after they become obsolete. I would say look to the online Apple store for Macbook Pros to be sold out, as the original iPhone was, before the update.
October - Early November - It would make sense for Apple to release their pro level notebook before the holiday season as well as Macworld. It is older than the Macbook and pro consumers pay more attention to the finer details of their machines as they need every ounce of processing power and available RAM they can get their hands on. Apple is more likely to leave Macbook upgrades until Macworld 09. This will enable them to release a consumer level product at a major Apple event as well as a pro product in the form of a Mac Pro.

On a personal note: DDR3 is going to be SICK!!!!
