I don't think Tom Welling will be typecast simply because the role he is playing in Smallville allows him to play a regular guy, for the most part. It's so much more than "Superman" or even the suit.
Welling was able to really stretch his acting chops in this role, more so than Cain or Routh. What I mean by that is that he is allowed to portray many different facets to Clark Kent (i.e., psychotic, romantic, angry, heroic, drugged, paranoid, etc.). His portrayal allows him to work outside the usual heroic, do-gooder, version of Clark Kent that we have seen in other mediums. Frankly, it's the main reason I started watching Smallville. It was much more dramatic than I ever expected it to be. I was expecting campiness or even cheesiness. Smallville is neither. Because this show is about his early years, it really is just like watching a regular drama in a lot of ways, except that the main character also has superpowers. I just think it would be different if he was in the suit all the time. For whatever reason, it DOES end up labeling an actor, imo. The Adam West analogy in Batman and the George Reeves situation in The Adventures of Superman, I think, attest to that fact.
In my honest opinion, if Tom Welling had been wearing the suit for six years on Smallville, I have no doubt he would be typecast at this point. The fact that he has never been seen in the suit and has played a much edgier and dramatic Clark Kent can certainly work in his favor in future projects.
On a side note: I think the smartest thing Welling can do when Smallville ends is get smaller parts in big films and build up a resume. He shouldn't come out of the gate in a big picture where he is the star because then the entire picture rests on his shoulders. Of course, I could be all wrong about it. Alot of it is just plain luck.
Bottom Line: No, I don't think Welling will be typecast. I think he will certainly be remembered for playing Clark Kent (or young Superman), and in my opinion, the very best portrayal I've ever seen, but I don't think it will typecast him.