I've been checking periodically, the regular (non-educational) Apple store was quoting 1-2 days for delivery last week, then 3-5 days for most of this week. This evening it changed to 5-7 days.
If I were betting I'd bet on the day after the Crystal Clear promotion is over (i.e. Aug. 13) as the day the new G4's are introduced, starting to ship on that date. The intensity of the rumors around the time of MWNY combined with the fact that they followed up a promotion on the G4s immediately with another promotion (rather unusual) leads me to believe that the new models were really ready before MWNY but Apple held off to rid themselves of excess inventory.
If they weren't going to announce next week, I would think the next most likely date would be one of the keynotes in Sept. (Seybold or the Apple expo in Paris). The fact that the ship date is streching out like this in the Apple store as well as the other stories of depleted inventories in this thread seems to indicate they aren't going to wait that long.
Unfortunately, not everyone has been able to sell their excess inventory; there is a Mac/PC store a block from my apartment that has stacks of G4 boxes in their front window! The same store sold out of their TiBooks weeks before the new model was introduced, but I guess it is harder to move those G4's.