Yup – I have considered Google Gears, but mainly for performance, not off-line work.
However, working off-line requires more. I have thought about this and I believe there is a good solution to it. Factor include having to store the changes you make until you connect again, then pushing those changes to the server. I have a bunch of higher priority features lined up ahead of this, so I haven’t done any in-depth thinking about it yet, except that I am comfortable that it is possible and on the future development plan.