We have released the file format specification for Flash 6. This contains everything you need to know to create your own SWF file to play within the Flash player, including information on the FLV video format.
Note, this is a specification and not an SDK (which we offered in the past). This means that the documentation is much better, but there are no sample files. This allows us to update the specification a lot quicker once future versions of the player are released.
You can find more information as well as download the spec here.
You can read the FAQ on the specification here.