According to a recent post on the EPIC forums by Gears of War 2 Executive Producer Rod Fergusson, you will be able to earn double experience when playing Gears 2 the weekend of July 4th. As an added bonus, the winning team also gets an extra 500 experience per player.
This weekend would be a great time to get those new achievements for leveling up online as I'm sure a lot of players probably haven't reached the highest level yet. It is also a good time to go for that achievement for being all waves of horde mode since the Bloodmounts have been taken out and replaced by squadrons of Tickers every 10 levels. I'd love to see a group of Tickers stand up to a Mulcher!
The changes will take effect Friday morning and end on Monday morning.