It seems that this year Google's most popular web applications will work offline. After Google Reader and Google Docs, two other services will integrate with Gears. Andrew Fogg saw a preview of the integration at Google's offices and found that Gmail and Google Calendar will add offline support in approximately 6 weeks.
In the past months, Google accidentally enabled the option in Google Calendar and some users saw the dialog illustrated below: "To view and edit the next 3 months of your Google Calendar when you're not connected to the Internet, click OK."
Andrew also found out that Google will add SyncML support for Gmail contacts next month. He thinks this is "related to the sync that they worked on with Apple for 3G iPhone". SyncML is a standard for data synchronization supported by a lot of companies. "SyncML is most commonly thought of as a method to synchronize contact and calendar information between some type of handheld device and a computer (personal, or network-based service), such as between a mobile phone and a personal computer."
Update: Andrew Fogg deleted the tweets referenced in the post, but I have a screenshot.
Another update (Jan. 2009): Much later than expected, Gmail gets offline supports.