You can now grab Cells3 and Celtk from the c-l.net Cells repository.
.\Celtk\ltktest-cells-inside.lisp has a load of doc inside it, including some on just Cells.
Check out load.lisp in either source tree to build and run. (Checking next that I remembered to commit those, speaking of which, I use neither those nor ASDF when working, so they often have bugs. Speaking of which...)
Bug reports or request for clarification on the doc exceedingly welcome.
Otherwise, I will be getting to work on a commercial app using both these (as long as Tk holds up, which looks good so far).
ken