They're $20—yes, about as much as a Blu-ray—and they'll pop up as the usual $5 HD rentals within 30 days after that. It kicks off with Quantum of Solace and Twilight for pre-order, with Transporter 3 (Jason Statham, YES) and other blockbusters available right now.
HD movies are pretty hefty—about 2-3x bigger than standard def, running between three and four gigs, and then you get a bonus file on top of that, the iPod- and iPhone-compatible standard-def version—so I hope you've got some sick NAS storage for these whoppers. But is $20 really worth it? [Apple]