What about Flash with the flashserver external? What's needed is a small toolkit to create flash documents with special widgets for PD. Some of us will complain that Flash is proprietary, and I agree, it's "wrong"... There's also the Tcl/Tk web plugin, but the release version is too old, and the cvs version don't work.
what about Python + Pygame (and maybe wxpython) ? Using OSC to comunicate... this is what i am experimenting on at the moment.