There is also the matter of the SIZE of the carry-on, as I noted in my contribution to this thread on the UA forum in November '03:

Funny you should mention this... we flew out of HKG yesterday with NW. All we had were carryons and the agent insisted on taking a look and then said fine. As we walked to screening the baggage police intercepted me because my rollaboard was "too large." By golly, it wouldn't fit in their template. I roared back to the counter to the same agent, told him my problem with some degree of asperity, and he produced a little coupon that another employee carried back to the baggage police to give me a "home free."

In fairness, we had never noticed before that my rollaboard is barely too long to fit in a 744 overhead lengthwise, but we still managed to share the overhead with a box of chirping birds on the outgoing NRT-HKG leg.

http://www.flyertalk.com/forum/showthread.php?t=110116