While Wi-Fi signals have been measured to travel 125 miles in non urban settings, they sure do travel a long way from building to building if the signals eminate from the upper floors of a building.

Also, it's important for anyone responsible for corporate or government Wi-Fi security projects to know that encryption, authentication and VPNs do NOT secure WLANs. In fact these protocols, along with WPA2 / 802.11i can be cracked, compromised, and circumvented.

Here is a webcast discussing the issues involved:
http://www.newburynetworks.com/events/whyencryption.php