While I agree with what you are saying Keith, this falls under the heading of not leaving additional money on the table if I'm able to pick it up as well. I don't feel entitled to the refund, but at the same time I don't wish miss an opportunity for one if it exists.
I found the answer on United's website:
http://faq.ua2go.com/display/4/kb/browse/cat.aspx?cid=1003&kb=1&L1cid=1003&level=2&
Basically, I need to see the fares drop more than $150 to realize a credit.
"When we introduce new fares or reduce current fares in a particular market, you may change a previously purchased ticket (purchased on or after March 20, 2009) to the new fare if:
the change is made prior to departure;
you do not change the flight, date, origin, destination or stopover points;
reservations are confirmed in an appropriate class of service;
all fare rules are met; and
the lower fare is not a united.com-only fare.
To change your ticket to the new fare, you may request a credit voucher (for a nonrefundable ticket) or a refund (for a refundable ticket) through the My Itineraries page at united.com or by calling United Reservations at 1-800-UNITED-1 (1-800-864-8331).
A $150.00 administrative fee will be assessed for domestic (U.S./Canada) tickets. Fees vary for international tickets. Any applicable penalties are retroactive to the new fare ticket."