I've never heard of this before, but it's absolute crap. I'd take five minutes and fill out the the FCC complaint form at http://www.fcc.gov/complaints (choose Wireless Telephone > Billing, Service, Privacy ... > Online Form). Send them back a letter with the bill (including the reference number that the FCC gave you). The correspondence will be handy if they ever try to cut you off or send you to collections.

It's amazing how Slamming (http://en.wikipedia.org/wiki/Telephone_slamming) refuses to die.