The mechanics and business of distributing software for the device is still under development.
In general, though you'll be able to upload a "widget" to our side and make it available just to yourself, your buddies, or submit to to us for approval to be exposed to the general public.
For lower-level hacking, all of the tools are open source. We'll be adding information to the wiki on how to obtain and use the tools and source code. Some of those links are already there.
We passed out a full SDK/HDK on CD at Foo camp, so anyone that has a chumby has everything.