The Source Film Maker is the exact same tools that have been used to make all of the "Meet the..." videos for Team Fortress 2 along with all the intro cutscenes in recent Valve games such as Left 4 Dead. Valve released a short demonstration video showing off how easy to use and how powerful these tools are.
The beta for these tools was announced though it's invite only at the moment, you can sign up and hope to get an invite at www.sourcefilmmaker.com where they've also more info. No date was announced for when the Source Film Maker would be available to all but one can imagine it'll be a few months after beta starts.
Having seen the demo video, these tools look amazing. Giving someone the ability to record gameplay, add models and mess with animation and lighting all on the fly. I can't wait to see the sort of things that can come out of this. If this could be combined with Garry's Mod, this would leaves every asset that Valve has created at the fingertips of the community to make movies out of easily. I'm really excited for what we might see out of this.