Me too - using Gmail + Gears w/ Safari 4 Fluid app, and a dock badge 'inbox count' userscript. Google will tell you you can't install gears with safari 4 - but it works.
It is available for Firefox 3.5 and it does sync all your recent mail, including attachments. It uses an automatic, non-configurable heuristic to determine how much mail to sync. I believe mine syncs the past 1 year's.
All my mail is synced locally but I can still use the Gmail interface.