You could rent 'em before, but starting today, you can buy movies in high def from the iTunes Store. More than that, you'll be able to buy HD movies before you can rent them.
AU: Seeing as how we can only rent HD through the Apple TV over here, I have no idea whether the purchase option has been introduced for ATV users. But you can't rent or buy HD through the iTunes Store yet - probably understandable given the state of broadband. - NB
They're $US20—yes, about as much as a Blu-ray—and they'll pop up as the usual $US5 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 $US20 really worth it? [Apple]