Share network info

I am working on studing Network security so I try different this to better understand how to secure a network. Right now I have a wireless antenna connected to my Laptop, and when I drive to or from work I have netstumbler running. When I find anyinteresting I check them out. Purely for educational reasons. I connected to one and recieved info on approx. 140 computers. I want to let the sys admin know but I am not sure how to do it? Is it ilegall to connect an scan someones network? This guy needs to know this info. I have been thinking about setting up an Email address like at yahoo to send this guy an email about this. How would you handle this?