Taym, Crashplan encrypts your data before it leaves your computer to go to their servers or even to go to another computer in your home. This page has more info.

The nice part about Crashplan is that if you do have a very slow connection, at least it also includes the ability to back up locally as well. The application then detects which method will complete the fastest, and starts with that one. That way at least you're safe from hard drive failure and any environmental scares that would be localized to the area of the original data (burst water pipe over your computer, thief takes your computer but doesn't see system in another room, etc).