Huh. Thanks for all your responses. Now I feel better about using these services for most of my files. And I downloaded TrueCrypt, thanks for the tip.
For off-site backup and file syncing, after your feedback I'll probably go with a paid SugarSync account because I don't need version control and I like the way it deals with folder sharing.
I'm considering signing up for MobileMe, too. It's expensive in terms of the GB limit, but it provides other services such as a me.com email account, as well as syncing mail, calendars, and address books, etc, across Mac and Windows platforms and devices. (I don't know if it works for Linux users.)