PC Gamers, we hate your guts. Console players, cry bitter tears for all the deadly testing you'll be unable to do. Valve has just released a beta version of authoring tools for Portal 2. And these aren't just blocks and buttons that can be put down in amusing ways, these are some of the same tools that they themselves use when making science for GLaDOS.
For Steam users, just go to your "Tools" tab and begin creating maps for single player, co-op and more. And while the tools rely on the Windows platform to use, Valve has stated that it might be possible for user created content to make it's way to the consoles.
For now though, it will just be those on the PC who will be creating new designs for all the test subjects out there. Experiment away, folks. Experiment away.