I've tended to think that the new PowerMac announcement will correspond to the end of the current promotion. The promotion ends the 12th so an announcement around the 12th or 13th would be plausible. It could be later as some have speculated, even into September, but I think anybody expecting it to be _before_ the 12th is engaging in some wishful thinking.
Well, here's some circumstantial evidence that points to it being around the 12th/13th time frame rather than later. Up to this week, if you went to the Apple store and went through the motions of ordering a PowerMac G4, you were quoted a 1-2 day shipping time. If you go there today, however, you will be quoted a 3-5 day shipping time. At least that's what I got when I tried it; I tried the low and high end models, without any custom features (which will automatically result in a longer shipping time).
Now, that quote is in business days, not calendar days (as my wife found out to her displeasure a few weeks ago) so five business days would be a week from today, the 12th. So this _could_ mean that they have really stopped shipping the current models and will wait till the new models ship to fill the orders (for this theory to be true they would have to ship on that date, not just announce).
A couple of points to elaborate further:
1) As I alluded to above, my wife made a recent purchase, a TiBook, from the Apple store. This order was placed before the new TiBooks were announced. We knew they were coming soon, since there were rumors and the local merchant's stocks were becoming depleted. I would actually have waited, but she needed it right away, even if it was the old model. Well, we were quoted a pretty long shipping time (something like 10-12 days) and as I said this turned out to be business days. As it turned out, the new model was announced during that period, and the old order was automatically cancelled and replaced with the new model. (We actually got a slight deal on it as we got it at the old price, and the new price was slightly higher).
So, when the shipping date on the vanilla model begins to stretch out like that, it _could_ mean that they are preparing to switch to a new model line.
2) It has been speculated that Apple might announce at a certain date and ship at a later date. I think this is unlikely when they do an announcement like this that is not timed to coincide with a keynote address or something like that. I believe (please correct me if anyone can remember a counterexample) that in all of the instances where they announced hardware that wasn't actually shipping, they have done it at a show where there is some reason to make the announcement in advance (in order to have something to announce at the show) Otherwise, they generally hold the announcement until they are ready to ship.
P.S. I don't think the Jaguar retail introduction is any constraint, I think they are fully capable of shipping Jaguar with hardware before the retail date. The rumor is 10.2 went GM last week which might be cutting it close for shipping it with the G4s if they ship next week. They might also just include a free "Up-To_Date" subscription or something.