If you haven't got round to securing your wireless network yet, check out this video that came from the BBC's "Real Hustle" programme.
The video makes some good points though when they approach the homeowners, I have no idea why they claim to be from Ofcom, since they proceed to just tell him everything they know anyway. Why not just say they were from the BBC? I've seen the show a few times and personally I've never quite believed that the interactions with members of the public are as straight forward as they seem.
But that aside, the video should demonstrate to you why you must secure your network. WPA is your best option, and WEP (128 bit) if you don't have that.