Well, we were so excited about the possibility of AJAX applications working even while offline, we went ahead and put together the Google API Library for Google Web Toolkit with support for Google Gears. This open source library allows developers writing applications with Google Web Toolkit to take advantage of the Google Gears APIs to develop applications that work offline — on a plane or anywhere else you happen to be.
Please keep in mind that this is an early prototype of this library, and so your mileage may vary. We are planning to add support for other Google APIs; if you'd like to help, please check out Making GWT Better, and contribute your ideas. If you're curious to get started, check out the project on Google Code.