Given the upcoming MMO, Warhammer Online, there has been a lot of discussion over the past year or so about who's ripping off who. As hopefully everyone knows by now, Warhammer (first published in 1983) has been around long before anyone even heard of the Internet. But what many don't know is that before Warcraft, Blizzard had been in negotiations with games Workshop to make a Warhammer game. For whatever reasons, the deal fell through, Blizzard modified a few things and released Warcraft (which is why the games seem similar in many ways). I guess there was enough ambiguity for Blizzard to avoid a lawsuit.
Anyway, in short, Warhammer has been around for a long time, Blizzard was undeniably, heavily influenced by Warhammer when they created Warcraft, and now, Warhammer has a lot of work ahead to claim its rightful section of the MMO pie.