Eavesdropping is a misdemeanor or felony + fines in most US jurisdictions.
Remember the name of the site you post this on.
Social-Engineer.org is ran by some of the Offensive Security / Backtrack team.
The article with the device dealt with a Red Team vs Blue Team challenge. The person who built and deployed the device in the blue team area, was a on the red team. There was nothing in the contest rules to say it couldn't be done.
It really showed what you can do with off the shelf materials and some solder. Gives an idea for things to look for on Physical security assessments too.
General Certification : CISSP ISSAP
Hadnagy : [Article]-An Insider`s Look at the Social-Engineer.Org SE CtF at DEFCON
Editor-In-Chief : Free Webcast: Don't Pick the Lock, Steal the Key - PW Auditing with Metasploit
Links to cool sites. : Ninja-Sec Challenge You !
Tools : Nmap 6 Released
