This just happened to me. I was booked into an L class ticket and was trying to do the $25 same-day confirmed stand-by to an earlier flight. I called within 3 hours and the Elite desk agent informed me that since she did not see L class availability I could not do the confirmed stand-by but would have to go to the gate to do so.

I got on the flight in the end, saving $25, but if this had been a busier flight this could have gone very badly. I checked out of my hotel and if I had not gotten on that flight I would have had to wait 14 hours for my next flight (assuming the other flight wasn't available either).

Per a post from CO Insider last year this should not have happened. If any seat had been available I should have been able to do the confirmed stand-by. When I got to the airport the TA had no problems putting me on the stand-by list and the GA released a seat for me as soon as I approached the podium.

Incidentally regarding the new system of filling empty seats from the back for stand-bys, I think this should take into consideration your elite status. If I have to sit in a middle seat due to travelling stand-by I should at least get to sit in the front so I can have a better chance of making my connection.

The confirmed stand-by is only available AT the airport. You cannot do it over the phone.

Uhh..

Same-Day Confirmed Travel via the CO website:

You may take advantage of this same-day change option when traveling on Continental Airlines, ExpressJet Airlines, Inc. dba Continental Express and/or Continental Micronesia. Changes may be confirmed within three hours of your new desired flight at any airport check-in kiosk or with an airport agent. You may also contact Continental Reservations by phone.

According to that post and what I have heard, they open all buckets at a nominal time before the flight (3 hrs? 12? 24?). Since most fares issued from those classes require advance purchase, people would not be be able to purchase as walkups on the day of. However, they would be available for people to change flights without problems.
I think one of 2 things may have happened...
Call center agent didn't know of standby policy or something--perhaps she was looking to purchase an L class day of, and it wouldn't show any L availability.
Buckets did not open up as scheduled.
As an alternate, maybe the information you found is incorrect or poorly worded.

Buckets open 3 hours before the flight.

The class has to be available, not "any seat". I recently wanted to change a early Monday 1am LAS to IAH flight to later in the morning (by phone, by the way). The agent said there were seats available on ALL flights later that morning, but the 9:30 flight had no seats in my original fare class. I did have my choice of 4 other flights, though.

I have resolved, however, to stick to the 1 am departure from now on. I could not believe the security line in LAS on Monday morning.