The great thing about web based applications is all your data is stored on a server instead of on your computer and the worst thing about web based applications is all your data is stored on a server instead of on your computer. As much as I love Gmail, Google Calendar, Documents and Spreadsheets and all the other Google applications I've been burned more than once by not having an important e-mail or document on my computer when I couldn't get to an internet connection. Google should really create a desktop application that automatically saves recent e-mails and active documents to your computer so you can get to your stuff when you can't get to an Internet connection, sort of like a briefcase that automatically grabs all your important stuff.
Basically, a Google Briefcase program should be able to automatically download recent e-mail conversations, calendar events, and active documents. Those are really the Google applications that loosing access too can be the most frustrating, although a tie-in with Picassa Web Albums could be really interesting. Users should be able to select the number of e-mail conversations, active documents, and date range that Google Briefcase automatically downloads. Downloaded e-mails should be able to include attachments. And it would be ideal if the all this data were stored in the most compressed format possible. For example, all documents could be stored in a highly compressed single file, and individual documents could be exported to a desired format when necessary. It would also be really handy if you could start new e-mails or documents and have them automatically upload when you come back online.
Lifehacker.com has a great tutorial on automatically backing up your Gmail messages if you have a Windows PC, but unfortunately it doesn't work for Calendar, or Documents & Spreadsheets.