Infinity Ward have announced that there's double XP up for grabs across all Call of Duty: Modern Warfare 3 playlists for a week from May 23rd (that's today!) through to May 29th.
The second Collection Pack for MW3 launched on Xbox LIVE this week, and includes three new Survival maps, two new Spec Ops missions, and two new Face-Off maps. You can nab it here for 1200 MSP. The double XP goodness should be available across all platforms, though.