Yesterday I was wondering out loud about whether World of Warcraft was being significantly worried by the free-to-play revolution, and now they're expanding the trial to have no time limit on the first twenty levels (which was my favourite bit of the game.) They're calling it the World Of Warcraft Starter Edition, and it's still limited by other trial aspects, such as not being able to use the auction house, and suffers a few other caps such as a limit of 10 gold. If trialees get to level 20 and want to open up their game and keep playing then it's possible to to purchase the Burning Crusade for $20 and subscribe for the normal amount. The Burning Crusade has also now been unlocked for free for anyone who owned the original game.
There's a detailed FAQ about the Starter Edition here.