Well, there's a big difference between being accessible to strangers at the IP level and at the MAC level. If I'm not on your subnet, then I can't see traffic that you're not sending to me. But if I'm on the same subnet (and especially if it's not switched) then I can eavesdrop. They're not just capable of seeing your computer across the network, they're seeing all the data coming and going as well.

One possibility is to have a non-wireless box act as a VPN endpoint, and then establish VPNs between the household wireless computers and it; that way anyone stopping by in the neighbourhood is free to use the network but can't see the traffic others are broadcasting.