I'm seriously quite excited about this. I doubt I'll personally be able to make anything, as heck, I failed at making anything decent in Garry's Mod, even. I'm still going to sign up for the beta and try give it a spin. What makes me excited isn't what I can personally do with it though, it is more what the right people in the internet community can do! I have honestly enjoyed the Meet the Team shorts (especially the new Meet the Pyro!) and would love to see what the community can make. :)The Source Filmmaker (SFM) is the movie-making tool built and used by us here at Valve to make movies inside the Source game engine. Because the SFM uses the same assets as the game, anything that exists in the game can be used in the movie, and vice versa. By utilizing the hardware rendering power of a modern gaming PC, the SFM allows storytellers to work in a what-you-see-is-what-you-get environment so they can iterate in the context of what it will feel like for the final audience.
Also; I recall someone here or some other source mentioning that the Meet the Team videos were originally Valve's experimentation of a full length film of sorts? I wonder if we won't see something awesome from Valve themselves in the future then? I sure hope so.
EDIT: I wasn't entirely sure if this belonged in gaming discussion, as it is in a way dealing with films. Apologies if I did misplace it.