According to Valve, the Counter-Strike: Global Offensive software development kit is being developed by Mike Durand, and will be dropping three months after the game is released. The SDK, of course, will allow players to mod the game as well as create new maps.
The SDK will also include some maps from Counter-Strike 1.6 that did not make the transfer to Counter-Strike: Source. According to Valve boss Gabe Newell on the Seven Day Cooldown podcast, both CS 1.6 and CS: Source have been integrated into Global Offensive "unless the beta testers are blowing smoke up our ass."
Counter-Strike: Global Offensive is due to be released on PC, Xbox 360 and PlayStation 3.