Portal 2 mod tools are out now
Valve announce that Portal 2 level creation tools are now available to all. The latest version of the Source Development Kit is in beta, but can still be used to create new single player and co-op maps, character skins, 3D models, sound effects and music for Portal 2. You'll find the free download in the "Tools" section of Steam.
If you're new to mapping, and haven't used Valve's mod tools before, you'll find a ton of useful information on the Valve Developer Community wiki. Valve also recommend that budding mappers join two mailing lists to keep abreast of all the latest information on updates to the Source Development Kit, one for general mapping, and one for Portal 2. Good luck!