pathfinder :

I've been experimenting with GWT, seeing how it can be hacked into unusual applications -- bookmarklets, Greasemonkey, widgets -- without breaking. The answer is that there are a number of things about the GWT runtime that break in these scenarios (Each bit of code loads in a different HTML file, so each has its own sandbox, therefore, no cross site action). I'm still looking at ways to make it work, but in the meantime, GWT can be used without any problems to develop Opera widgets.

Using GWT to Write Opera Widgets