Quick update: Here's what Cassandra Clare said on her website about the casting of Alex Pettyfer as Jace:
I wanted to wait till I could have an official chat with the studio folks before I said anything, so my small contribution is: I know Alex has met with the director, Scott Stewart, several times; I know he screened Scott's movie, Priest, and loved it; I know he read the TMI books over the holidays and really liked them and liked Jace, and his agents are representing him in negotiating making a deal for the role of Jace. At this point it's all about the deal, which really doesn't have anything to do with me, so I stand back and wait for news like the rest of you.
So fingers crossed!
Did anyone see him on Ellen? He even took his shirt off!