Maybe something to look at for Xfce 4.6?
Here’s a post by Ryan Lortie, who has been working on a (possible) new API for GNOME applets. The work was done as part of the Google Summer of Code and it looks very interesting.
It would be kinda cool if we could support the same API for Xfce 4.6. Most of the GNOME dependencies seem to have been dropped… or maybe, by the time we release 4.6, we simply switch to using the GNOME panel, who knows?
Or maybe I should be nice to our plugin writers and not change the panel API every major release :-)