This was announced back in December 2009.
http://www.dot.gov/affairs/2009/dot19909.htm
The actual published regulation is available at
http://www.regulations.gov - On this page, enter "DOT-OST-2007-0022-0262" in the field
Enter Keyword or ID and click
Search.
Per the above, the publication date was 30 Dec 2009, and the effective date was 120 days after publication, or 29 Apr 2010.
Sec. 234.11 Disclosure to consumers.
(b) For each domestic flight for which schedule information is
available on its Web site, including domestic code-share flights, a
reporting carrier shall display the following information regarding the
flight's performance during the most recent calendar month for which
the carrier has reported on-time performance data to the Department:
the percentage of arrivals that were on time--i.e., within 15 minutes
of scheduled arrival time, the percentage of arrivals that were more
than 30 minutes late (including special highlighting if the flight was
late more than 30 minutes of scheduled arrival time more than 50
percent of the time), and the percentage of flight cancellations if 5
percent or more of the flight's operations were canceled in the month
covered. The information must be provided by showing all of the
required information on the initial listing of flights or by showing
all of the required information via a prominent hyperlink in close
proximity to each flight on the page with the initial listing of
flights.