I had a tip off today that August 22nd is when Microsoft is scheduled to finish the first batch of "Jasper" motherboard powered Xbox 360. This are going to use a newer cooler 65 nm GPU as well as a 65 nm CPU. They are expected to hit the stores on the first week of September 2008.
I can't totally vouch for the source of this particular tip off but it seems to jive with what I've heard elsewhere.
Now this is just when they are going to switch to the motherboard design. The still could change the bundle (Like changing the 20 gig hard drive on the Pro system to the 60 gig one that they have been working on) or change price between now and then.
I can't wait to see when someone figures out that the Xenon Xbox that they sent off to get RRODed repaired AKA "RnR" was sent back with an "Opus" motherboard. That is likely to be identified by it using the same power consumption as the "Falcon" system but in a "Xenon" case.
Microsoft is probably not going to be making a big deal about the switch to the newer motherboard design but since the "Jasper" is going to be using a new GPU chip it is going to be using less power and can probably be identified by the power consumption rating on the box.