The NY Times has an article about hackers that ripped off (they believe) Net to Phone and other VoIP companies (News.com version here).  It makes for interesting reading.
It appears that the plaintiffs gained access to the routers of an investment firm that had accounts with some of the VoIP carriers, then routed traffic through this path and terminated it on the PSTN.
They then resold the resulting termination, earning over $1 million.  Now, for a 23 year old immigrant to master the telecom game to be able to sell such termination is clearly indicative of drive, intelligence, and competence.  It’s a shame that ethics and honesty weren’t part of the package.
 If you run VoIP, you better keep things tight. VOIPSA is a good place to start.
It was quite obvious it woul happen soon or later