WoW will definitely 100% certainly have a monthly fee. Its in the FAQ on the site. And it won't be on battle.net per se, although that may be how it gets accessed and where accounts are stored.
The reason they have to charge upkeep is because as an MMORPG, WoW will be constantly changing, new quests, new monsters, even new areas opening up. Meaning, they have to pay people to code in all this new shit, so they gotsta charge us for superior service and the like. Its the future of gaming, some say...