Update:

I received a response from CO Customer Service. There has been a change in policy, and certain fare classes cannot fly stand-by on an earlier flight on the same day without paying a fee. I was flying on an N fare that day, so I was ineligible. Here is the text of the note:

Dear xxxx:

Thank you for contacting Continental Airlines with regards to
confirmation number xxxx. I apologize for the delay in receiving a response to your e-mail.

After reviewing our internal records it appears that the ticket you
purchased had restrictions set in place by our new Same Day Change and
Stand-by Policy. The ticket you purchased did not allow for stand-by on
the return and only for confirmed travel, which is why the airport agent
was going to charge you to make those changes. If you would like to
review the recent changes to our Same Day Change and Stand-by Policy you
may refer to our Web site www.continental.com.

I have added your comments to our monthly Customer Service Report (case
ID number xxxxx) that is shared with our Airport Management team to
ensure that your concerns do not go unnoticed.

xxxx you are a valued Silver Elite member and, again I regret the
negative impression this may have left with you. We appreciate your
e-mail and hope to see you on a future Continental Airlines flight.

Regards,

xxxxx
Customer Care Manager

Such is life in the new world of air travel.

I cannot find the policy on CO.com, however. Perhaps such policies will be easier to find on the new site when it is rolled out.