Hi, Merv,
It's not all that rare for folks with wireless cards, so conceivably somebody using their Q or Treo as a modem could encounter it. Getting the LETTER is quite a topic for discussion here:
http://www.evdoforums.com/
But Verizon's being more upfront about informing folks that they will consider going over 5 GB as being indicative of the user violating the terms of service. I suppose some folks out there have busted 5 GB doing email, webbrowsing, and VPN, but I've never come close myself.
Lamar