The MW2 beta opens up to all platforms on Saturday, September 24 at 10 am PT / 6 pm BST and ends on Monday, September 26. Starting Saturday, anyone on all available platforms can download and play in the cross-platform multiplayer weekend.
Update: the Modern Warfare 2 beta is now finished. The end time of the beta was extended by 2 hours, but it came to an end at 12pm PT / 3pm ET / 8pm BST on Monday 26th September.