If I were Dan Hesse and Sprint....
This would be "part of the plan". Everyone is spot on. HTC and Google are best friends and the updates to Sense likely are already done. It'll probably push to the Nexus today at I/O and get a bunch of good "tech" press (read: free advertising). Then, oh, just a few short weeks later, BOOM! it pushes to the Evo on activation as speculated = delighted customers, raving reviews by tech blogs etc (I know they're already coming out using the current build...having 2.2 is just reason to "review" it again, eh?), and get a ton of good free press again from all the digital channels.
I think this is an ace in the hole and I'm optimistic. They'd be very very smart to make a huge splash on all fronts with this product launch, and 2.2 at launch is an easy way to make that happen. Ironic they're launching the phone 2 weeks after Google I/O, and just in time for Premier customers that bought Pre's to upgrade??? I think not. Sounds well planned out if this is how it truly goes down. +1 to you Sprint for product launch strategy.