Ok, this isn't exactlty phone phreaking, but it does involve telephones, telephone security, and history. John Young's Cryptome site just posted a formerly classified 1955 FBI/DOD document concerning a pretty interesting thing: a technique for bugging a room using an on-hook telephone by sending ultrasonic frequencies down the telephone line. From the document:
The entire thing was classified TOP SECRET. Apparently more modern telephones, such as the 500-series phone, were not vulnerable to this technique. You can read the full details here.
Speaking of "more modern" phones, makes me think of this.