"\"T\" Tung" wrote
I hear a lot of my buds say that FAR Part 61 allows a pilot
holding an ATP to instruct another pilot towards a qualification
or a rating in an airplane without having a CFI ticket. I say
that the ATP-holder can only do that if he's doing it as part of
an air transportation provider (Part 121, 135, etc).

You are correct in your understanding.

Section 61.167: Privileges

b) An airline transport pilot may instruct --

(1) Other pilots in air transportation service in aircraft of the
category, class, and type, as applicable, for which the airline
transport pilot is rated and endorse the logbook or other training
record of the person to whom training has been given;

(3) Only as provided in this section, unless the airline transport
pilot also holds a flight instructor certificate, in which case the
holder may exercise the instructor privileges of subpart H of part 61
for which he or she is rated.

Bob Moore ATP/CFI