They say, "This means that all you need to do to turn the Raspberry-Pi into a (ridiculously powerful) FM Transmitter..." and claim it's transmitting at least 50 meters away, which is far beyond what's allowed by the FCC. Not that they'd care unless someone filed a complaint.

Wrong.

There are multiple rules for different spectrum.

Anyone can transmit for miles on FM in the 'green dot' / 'blue dot' spectrum. The laws were written for business, but anyone can use them.

http://en.wikipedia.org/wiki/Business_band#Overview

But that's not the band on which this is transmitting.

This can be configured in software. Pi can transmit up over 450 Mhz.

And with a paperclip antenna, it's leaking harmonics all over the place, not just at 450MHz or 100MHz, or whatever.

They describe they had to get several concrete walls and a metal cabinet between themselves and the Raspi before reception stopped, so I'd say too high.

A license holder would have to complain to the FCC. If you jammed a commercial station they would try to track you down and seize your gear.