It seems they are to offer it free up until level 20 according to this BBC article:
BBC News - World of Warcraft offered 'free'
There some stipulations though. You can't join guilds or collect more than 10 gold coins, but you can have unlimited characters. Blizzard say it is to make it easier for new players to join in, however I wonder if it is that there are a lot more MMPRPGs out there now taking a bite from 'their' pie?