I have an application built with GWT/Appengine/Jdo...and i am using Google User Service for authentication.
Google Chrome Extensions can use OAuth...I don't really undestand OAuth yet..
Would GWT RPC have to be reworked to enable OAuth?
Is there another way to authenticate users for Chrome Extensions?
I would take a look at what the gwt-gae-bookmarks project does in this area. It's a Chrome extension written in GWT that interfaces with an App Engine app (requiring authentication) to store bookmarks.
It doesn't cover OAuth, but if you plan on having your users log in using their Google Accounts, then you don't need OAuth.