Eventually, yes. But what it is and when it will come, no one knows.
The biggest obstacle when another MMO comes out is the momentum that WoW has built up.
Right now "everyone" plays WoW, and the social ties with those who still play keep other players there. Why would you want to go off and play some other game that's just as good with people you don't know when you can play WoW with friends you've had for X years. Plus there are more people playing WoW than any MMO ever before (even people who were previously not gamers at all!).
Eventually people will get tired of playing WoW. The game has always had a huge churn rate... something like 4-5% (basically every month 4-500,000 accounts are cancelled), but Blizzard's numbers has generally gone up, which means for every one account that cancels per month, another one or two are created or reactivated. Eventually the churn rate will overtake the new/reactivated account rate
Other small MMOs coming out also take their toll. LotRO, AoC, WAR did this and the forthcoming new Star Wars and DC MMOs will draw players away, which will help contribute to the churn. They may not topple WoW, but even if only 500,000 players leave permanently, that's still 500,000 less players than WoW had before.
Also, Blizzard is currently working on a new MMO (this was said at BlizzCon '08)