The regulation changed in late 2009, so you may be looking at an older version of the regulation. Here is the current regulation. The "short" answer is a current state or federal issued photo identification.
§ 61.3Requirement for certificates, ratings, and authorizations.
(a) Pilot certificate. No person may serve asa required pilot flight crewmember of a civil aircraft of the United States,unless that person--
(1) Has a pilot certificate or special purposepilot authorization issued under this part in that person's physical possessionor readily accessible in the aircraft when exercising the privileges of thatpilot certificate or authorization. However, when the aircraft is operatedwithin a foreign country, a pilot license issued by that country may be used;and
(2) Has aphoto identification that is in that person's physical possession or readilyaccessible in the aircraft when exercising the privileges of that pilotcertificate or authorization. The photo identification must be a:
(i) Driver's license issued by a State, theDistrict of Columbia, or territory or possession of the United States;
(ii)Government identification card issued by the Federal government, a State, theDistrict of Columbia, or a territory or possession of the United States;
(iii) U.S.Armed Forces' identification card;
(iv)Official passport;
(v)Credential that authorizes unescorted access to a security identificationdisplay area at an airport regulated under 49 CFR part 1542; or
(vi) Otherform of identification that the Administrator finds acceptable.
