Blizzard published a new FAQ on the upcoming Rated Battlegrounds feature which will be released within the Cataclysm expansion. The Battlegrounds will be open to lvl 85 players and will offer the chance to gain top-level PVP gear through rating.

Q. What are Rated Battlegrounds?

A. Like the Arena system, Rated Battlegrounds are an alternative way for players to battle against opponents of similar skill levels and accrue rating to purchase powerful PvP gear. Instead of competing in a 2v2, 3v3, or 5v5 bracket, though, you’ll be testing your mettle in familiar environments like Arathi Basin and Eye of the Storm, as well as all-new Battlegrounds set to release in Cataclysm.

Q. How are Rated Battlegrounds structured?

A. Rated Battlegrounds are broken down into two brackets: 10v10 and 15v15. These brackets rotate weekly and can be viewed in-game via the calendar by activating the Battleground holiday filter.

Q. What happened to the 25v25 bracket?

A. We decided to remove this bracket. We don’t feel it’s feasible to ask players to coordinate a group of 25 players for Rated Battlegrounds. If any player happens to drop group just before a match begins, the raid group would not be able to enter the battle. We anticipate most players or guilds will stick with fairly dedicated 10-player groups, and making the jump to 15-player groups shouldn’t be too difficult.

Read more here