The remaining combo box bug has been fixed in the Ext GWT 3.0 Release Candidate. Please upgrade your copy of Ext GWT and try your test case again. While we're confident that we've addressed this entire issue, please reply if you notice any continued problems with combo box or with setEnabled after upgrading. Again, thanks for taking the time to report, investigate, and discuss this bug.
But there is still an issue you may have overseen: key handler are still fired. So for example take a TextField, add a KeyPressHandler and disable it. You can't really type anything (as expected), but the event is still fired. I'm not really sure about that behaviour.