I don't think they'll view it as a problem if two MMOs don't work side by side. They'll just support WoW as long as people continue to play it, and allow for the gradual ramp up of players on the new game at the same time.
Since they haven't announced the game yet, and aren't scheduled to appear at E3, there must be quite a lot of development time ahead. Assuming the release is still two years out (reasonable if they intend to release SC2 and D3 first), a time frame of 2010 will be a full 6 years after the release of WoW. That's longer than the lifespan of most gaming consoles, and Blizzard should be extremely satisfied with milking a gaming "generation" out of it.
Presumably they'll want to come up with another MMO that can also maintain a fan base for an entire gaming generation, so that type of a thing can't possibly be quick to build. They'll need an engine that can run on today's new computers (which will be two years old on release day), but the engine will need to be considered acceptable 7-8 years from now as well. They've got a great team, so I'm sure everything will turn out quite well.