I've said before that wireless networks are not secure. You can do certain things to increase security to a large extent, but even with that added protection, certain "secure" wireless networks can still be hacked. In about three minutes.

http://www.tomsnetworking.com/Sections-article111.php

Granted, this was done by the FBI. If anyone can do it, they can. But the technique and tools are available to anyone, so it doesn't hurt to invest in additional security like local encryption and (as always) backups. Click to read more about encrypting your files or the differences between backup devices.