I wouldn't have a problem doing it. However, if you are uncomfortable, look for a new job and quit when you have found one. You seem to be in the field a lot, so you should be able to schedule some interviews during your away time.
I wouldn't tell the companies that are getting the wireless networks anything about security or the like. What I would probably do is maybe start a new company that does security consulting like suggested earlier. If you are really on the ball, make some good snail mail packets that briefly explain some security issues involved with wireless and non-wireless networks. Let them know you can help tighten things up or reccomend other solutions. Again, never bad-mouth the people that actually pay you to install these things.
Also, send it to all the firms around you that you've serviced or could be potential clients. Don't single out just folks you done, mail everyone in the area.
That might help clear your conscious as well. I mean, if you did mail them offering to help, at the very least you've outlined potential problems for them and given it to the on paper. I wouldn't feel too bad after that.