Russ, I think this is GREAT. I think we could easily combine the 'stand-alone' version of the adapters with the adapters themselves. In a sense you can even combine them -- I'm sure in time many people will be very happy dropping a DLL that adds functionality rather than adding a bunch of CS files.
As an aside, I actually integrated a community-provided fix to a CSS adapters bug in my code base, but I didn't check it in to CodePlex yet. (The fix was from here: http://forums.asp.net/thread/1518559.aspx).
I think the best solution is the morphing one -- there should be "one" CSS Friendly project, not multiple ones. Either both can be combined and the "standard method of distribution" would be the precompiled DLL but with source code available (similar to the Ajax extenders), or the precompiled version can simply be an alternate method of doing the same. I'm actually a fan of the former, and not out of ownership reasons, but because I would think that most people who use the CSS adapter code don't modify the code files -- and if they did, they should contribute their improvements to the shared source project.