Lets say for a moment that you live in an appt. complex and are running a wifi network. I know there are alot of security issues about wireless, but im going to be using it soon enough. How hard would it be for someine else in my complex to leech my bandwidth (jump on my network) from next door possibly?

I've done what reading I can and plan to use NetStumbler to see if anyone else is using it around me that I can tell. But just to easy my paranoid mind, what would one do to protect and leech bandwidth from a neighbors wireless network? The only real information I can find is wifi sniffers...