No, I haven't actually heard of one happening. That doesn't mean I don't think one could be very cool, especially after seeing this video of all the voices Chris Metzen has done for the various Blizzard franchises over the years. The idea that one man is the voice behind Thrall, Varian, Ragnaros, and other luminaries got me to thinking that maybe a film isn't the right way to go with the franchise. Now, of course I know about as much about making movies and managing beloved franchises as I do about installing thermonuclear reactors in submarines, but I'm not going to let that stop me from speculating about how awesome a Warcraft animated movie or series could be. (I also admit I've been watching a lot of DC Animated Movies lately, and the tenuous Blizzard - Wildstorm - DC connection has me all aflutter.)
If done right, a Warcraft cartoon has a lot of potential. I think quite a few fans of animation could point at various styles that would work for the franchise... I would personally stay away from the overtly computer animation of a Clone Wars (unless we could somehow get Pixar to do it. I mean, while I'm dreaming) and would try to go with something more like the actual game, with deliberately exaggerated and unrealistic lines. But I'll leave it up to you: who should star? Who would you choose to do their voices? What art style, what kind of direction would you take it? The sky's the limit for the Warcraft animated series, because as far as we know it's not going to happen.