There's a rumor going around that Australian actor David Wenham (who you'll probably know as Faramir in Lord of the Rings, though he was also in 300 and Van Helsing) is supposed to play Arthas in the upcoming Sam Raimi-directed Warcraft movie -- but not so fast. The whole thing appears to stem from a listing over on the Internet Movie Database's listing for Warcraft (which, by the way, seems to have a working title -- a few days ago, it was just called "Untitled Sam Raimi Project"), where Wenham's name was inserted as rumored to play Arthas. That in and of itself means nothing at all -- the IMDb can be updated and edited by almost anyone that comes to the site, and while it could be that Wenham's agent is trying to get his name out there for a certain part, it's just as likely that some overzealous Lord of the Rings fan is trying to do some casting of their own. Wenham's name under there as "rumored" has about as much validity as a mention on Wikipedia: not much.
Oh, and there's one more issue -- as discussed in this forum post, Arthas himself probably won't even be in the Warcraft movie, if what Blizzard told us at BlizzCon a little while back is to be believed. Back at the BlizzCon 2007 movie panel, we were told by the panel (including Chris Metzen) that the movie is supposed to start a year before WoW begins, which means Arthas is sitting up in Icecrown doing a whole lot of nothing. When fans specifically asked about Arthas, we were told that while the movie would certainly reference him, it likely wouldn't focus on him at all. So while Wenham (or his fans) may be angling for the part of Arthas, even if he got it, he probably wouldn't get much screen time anyway.
Of course, that's assuming that what we were told two years ago still holds. Gary Whitta, who worked on a screenplay for the film, says that Sam Raimi has his own ideas about the movie, so, as BlizzPlanet points out, maybe he wants Arthas front and center. But for now, the David Wenham rumor is still just a made-up rumor, and a really thin one at that.