I just noticed some glitch on Google Calendar and after some searching in page source code found out that offline functionality is probably coming soon. It can be enabled by changing javascript variable _sip to true.
Here's a screenshot:

xs227.xs.to/xs227/08186/gcalof ...

That "Learn more" link from the screenshot above leads here:
google.com/support/calendar/bi ...

blogoscoped.com/forum/112044.h ...
blogoscoped.com/forum/129400.h ...