Don't worry Doc, you are no more open to viruses through their router than your own.
As long as yours has a wap key password you should be fine.
But see this
http://news.bbc.co.uk/1/hi/magazine/6960304.st m
I have a deal with my neighbour, we share our wap key info (yes i trust them, and vice versa) Just in case we lose our connection.
But if the fool has been warned that it's unsecure and done nothing to rectify the situation, more fool them. They could suffer alot more than you downloading a movie.