The show is not cancelled, not yet. It's officially on hiatus, and the final word on its pick-up or cancellation likely won't come until Season Five (11 new episodes) finishes its run this summer. USA Network, which airs the show (which is produced by Lionsgate and Paramount), is still holding onto its option for a sixth season and will likely make its final decision then.
The situation came about by shooting all 23 episodes of Seasons Four and Five continuously last year. Normally, the show and the actors would've gone back into production about five months later. But the decision to wait to air the Season Five episodes this summer pushed back the timeframe that USA Network wanted to back its decision -- after all the new episodes have aired. Season Four's ratings were fantastic -- the show was the number five show on all of cable and, along with The 4400, helped USA Network dominate Sunday evenings. There is every reason to believe that this tandem will succeed again this summer, and that they could both be renewed again. The overall tightening at NBC Universal won't last -- they'll be back on top in no time. They've got a lot of great stuff in the pipeline, from NBC to USA and Sci-Fi.
But this means that, for the moment, the show's actors are not under contract and can join new series pilots or anything else they choose. But nothing is certain yet, and even if an actor were to take another job it wouldn't preclude them from still appearing on the show. Jennifer Finnigan did this last season, and many actors do it all the time.
But rumors of The Dead Zone's demise are far from true at this point in time. Everything will depend on how the show performs this summer.
So, there you have it. Don't give up on The Dead Zone just yet, OK?