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.