Sadly, the ultimate answer appears to be "very reluctantly" but it's an interesting read anyway. J. Allen Brack talks to G4 about WoW, consoles, and the barriers to entry for an MMO. The usual suspects apply, of course. WoW was designed for keyboards, consoles tend to have a short lifespan compared to the development time of a MMO, and WoW currently takes up about 15gb worth of memory while consoles like an XBox 360 have at most 20gb worth of hard drive space without buying any additions. This is no surprise, of course... they've said all this before.
What I really found interesting was that this is supposedly something they think about a lot. "In the case of WoW, we talk about it all the time." I'm the curious sort, so I find myself wondering why, given all the reasons Brack himself lists. Personally, I'd love to see a single player Warcraft property of some sort for consoles, perhaps a God of War style action game or even a Dragon Age/Mass Effect hybrid RPG. And who knows, perhaps Blizzard will come up with a way to do it despite their laundry list of reasons why they won't, they've surprised me before.