I'd put Halladay, Hernandez and Clement in roughly the same tier, with D'Amico and Tomko being similar: high upside starters who haven't always played up to their potential. I agree with petben's comments about Hernandez and Clement, but I don't think Halladay stands above them yet. In the past month, he's pitched a great game against the tough Yankees (in New York, no less), had two absolutely awful outings against the Rangers (even if he didn't have great fielding behind him), excellent games against Seattle and Oakland, and a pretty weak outing against the A's in which giving up only one run didn't reflect the way he pitched. I think he'll continue playing this way: he'll throw some gems, but also some stinkers.
Then again, maybe I just still have his earlier control problems in the back of my mind and am trying to come up with reasons to avoid him. That last outing did bring back memories... Yes, that's probably it. Ok, after reflection, petben's right: it's close, but Halladay probably should be spared the boot (grumble grumble).
Score one for petben!