As of Oct 1, 2011 he is Jace Wayland! IMDB incorrect
So I asked Cassandra Clare (author of the books) why Jamie's name was gone from the role of Jace. She replied with a link to her tumblr blog and said Jamie is still Jace. IMDB casting information is not accurate on the Mortal Instruments page as of October 1st, 2011.
You can read her blog here... http://cassandraclare.tumblr.com/post/10819911344/aw-jamie-looking-cute-a-lot-of-people-have
I for one am happy he still has the role! I would rather have a good actor than someone who just looks the part. Apparently Jamie's audition tape made the writer cry. I think he will be great in this part. Maybe just needs to put on a few pounds of muscle.