Yeah, it'd take a few minutes to not see him as Ash, but I think he'd be an excellent choice. I'm also glad some others have suggested Hugh Laurie, as I think he would be a great Roland Deschain, particuarly the look of his face and eyes. He looks like he has been questing for the Tower his whole life. It'd also be an interesting turn to go from playing the mega-sarcastic House to playing Roland who doesn't understand sarcasm. The last time I re-read the series this spring, I pretty much envisioned Laurie as Roland and it worked perfectly.
A few other casting choices in my personal version, Eric Bana as Eddie. I used to think Tim Roth, but he's a bit too old now to play a 21 year old. I think Jamey Sheridan did a great job as Randall Flagg in the tv "The Stand" and would prob want to see him again as Flagg/Walter [perhaps with better hair than the mullet in the stand].
Truth be told, if I had my choice of how it would be presented as a series of films, I'd prefer an animated version, perhaps based on the artwork for the comic series.