This phrase was made famous by courageous United Airlines Flight 93 passenger Todd Beamer, a 32-year-old Oracle Inc. executive, Sunday school teacher, husband and father who, along with Mark Bingham, Thomas Burnett, and Jeremy Glick, led other passengers in fighting terrorists for control of Flight 93, out of Newark bound for San Francisco, before it crashed into a field in western Pennsylvania, in a remote strip mine area in Stoneycreek, Somerset County, about 80 miles southeast of Pittsburgh, on September 11, 2001.
In the days following September 11, 2001, a patriotic fervor the likes of which few had ever seen gripped the United States. With "Let's Roll!" becoming a rallying cry for United States warfighters everywhere, the United States Air Force laid down a new policy regarding nose art on its aircraft. In recent years, the USAF had been very restrictive as to what sort of nose art could be applied to aircraft, and in the few years since 9-11 their policies have not changed. This was the major exception to the rule.
The nose art design depicts an eagle soaring in front of the U.S. flag, with the words "Spirit of 9-11" on the top and "Let's roll!" on the bottom. The design was created by Senior Airman Duane White, a journeyman from Air Combat Command's multimedia center at Langley Air Force Base, Virginia. The Thunderbirds and other Air Force demonstration teams have applied this nose art on all aircraft, while major commands and wings were authorized to apply the nose art to one aircraft of their choice.1
The Air Force and the Army Air Corps before it have utilized nose art throughout much of their history, and for a variety of reasons.
The "Let's roll!" nose art is being used to continue the remembrance of the events of September 11, 2001, spur on the nation's current patriotic spirit, and pay tribute to the heroes and victims in the war against terrorism.
1: I know of at least two USAF units that have removed the "Let's Roll!" nose art for some reason or another. I theorize that this number will increase in the future.