In case you haven't seen it already, Vanity Fair has a special photo feature paying tribute to Hitchcock by the somewhat bizarre method of shooting imitations of famous scenes from his films with modern day actors in them. It's a fairly silly tribute, if you ask me, but it makes for fascinating discussion. You can see the full series here.
Personally, I can't say their choices are all bad. I happen to think Jodi Foster in Birds would be a distinct improvement, as would Naomi Watts in Marnie (it occurs to me, btw, just looking at that photo, that the perfect person to play Marnie may actually be Nicole Kidman - after all, you have only to look at Ms. Kidman to know that something traumatic happened to her as a child), but this may be because I think anything short of a block of wood  would be an improvement on the mannequin that is Tippi Hedren. And I can certainly see Keira Knightley in Rebecca. Admittedly Ms. Knightley can't really act, but then Joan Fontaine can't either, and Ms. Knightley is both more spirited and much better looking. I'm even willing to live with Charlize Theron taking on Grace Kelly's non-role in Dial M for Murder, though I draw the line at letting either Scarlett Johansson or Gwyneth Paltrow replace her in Rear Window and To Catch A Thief respectively. As for Rene Zellweger in Vertigo, the very idea of letting that woman onto the set of a Hitchcock film makes my blood run cold, let alone actually giving her a part in one .
On the men's side things are not so bad. I like the idea of Josh Brolin in Lifeboat and I can totally see Javier Bardem carrying off Jimmy Stewart's role in Rear Window (and that's a serious compliment, though it's hard to imagine a role Bardem couldn't carry off) except that I think the sight of him staring out of a window unable to move will bring back too many memories of The Sea Inside. You're likely to end up spending more time worrying about his death wish than what's happening in the apartment opposite. The role they should really use Bardem in is Olivier's role in Rebecca.
It is, of course, entirely impossible to imagine anyone, and I mean anyone, replacing Cary Grant in anything, but I can just about see how Robert Downey Jr. could pull of To Catch A Thief. The idea of trying to have Seth Rogen (who in case you're wondering is the guy from Knocked Up) substitute for Grant, on the other hand, is utterly ridiculous. It would be like replacing a bottle of champagne with mango Rasna.
 Actually, even a block of wood might be better, remember that Monty Python sketch?
 I'd say "act in one" but there's no risk of that, is there?