Just so that people can stop emailing me one at a time ;) ... I grabbed the Google Gadget code the day it was released and spent some time that evening looking at it. It's written in C++ which is cool; on the other hand there's also well over 100,000 lines of code in there too. Much of it seems to be about replicating things Plasma already does for applets (list them, load them, etc).
There seems to be two routes to take, in any case, to get Google Gadgets into Plasma:
a) Link the code they've released into a Plasma::ScriptEngine plugin. This ScriptEngine would need to provide a subclass of the Host class and bridge that to QGraphicsView. It seems really bulky, though, and I'm not sure we'd get overly terrific integration with plasma. It may be fast to do, though.
b) Grab their custom JS hooks and, as we have with MacOS Dashboard Widgets, provide a PackageStructure plugin and then throw the content of the gadgets into a Plasma::WebContent widget (a Webkit renderer in a QGraphicsItem). Depending on what all is in their JS, this may actually be even faster.
I'm a bit busy to do this myself, but if anyone is interested in giving (b) a try, I'd be happy to first point you to the >MacOS Dashboard PackageStructure and ScriptEngine and then help you with an implementation for Google's Gadgets. How hard can it be? =)
Oh, and if you were trying to load MacOS Dashboard widgets in KDE 4.1beta1 it doesn't work: a commit made following the API review from Tokamak inadvertently broke Package installation for those guys. I fixed it today after one of our oh-so-valuable beta testers filed a bug about it. Huzzah for beta testers! =)
update: looks like we were all beat to the punch by the google gadget hackers themselves. i guess we'll have google gadget support sooner than i could have hoped for. rock on. =)