An obvious place to use Caja is, of course, in OpenSocial. So, a bunch of us at Google have been experimenting with this use case and the first outcome is an update to the container sample which allows you to try running your gadget Caja-ised (gotta think of a better name for that). We even have instructions on how to Caja-ise your gadget.
We haven’t tried many gadgets yet, but the good news is the example gadgets worked with (almost) no change. It seems clear that more complex gadgets are not likely to survive without at least some change but we don’t yet know how hard that’s going to be. Feedback, as always, welcome! And don’t forget to join the mailing list to discuss it.
 Right now, because Caja-ised code gets pushed into its own sandbox, you have to export any functions that need to be visible to the rest of the page (for example functions that get called when you click a button) – right now, you have to explicitly perform that export but we expect to be able to remove that requirement.