If you type javascript:_olp_showPromo(); into your browser, you get prompted to install gears for calendar. It suggests you will be able to view 3 months worth of calendar entires when off line.

Does it work ? Well, I can still see my calendar entries when I disable my internet connection, and can view various days by clicking on the calendar, but no idea if I could have done that prior to the gears instal. The usual "offline" icon that appears in reader etc doesn not appear on calendar.

Coming soon I think. No idea when.