David Kirkpatrick has an interesting (and unsettling) article in Fortune Magazine, titled "Identity Thieves Spying on Your WiFi?" where he thinks someone has been tampering with his computer files.
Every time I boot up my laptop at home in my suburban neighborhood, it registers no fewer than three other networks. One of them hasn't bothered to encrypt his network, so if mine goes down, I use his. The other night, I took out my laptop, drove around the block and found almost a dozen networks, of which 4 were unencrypted.
So, check yours to make sure you haven't left yourself wide open.