On January 11, 2000, about 2151 hours Pacific standard time, America West Flight 2744, a Boeing 757-2G7, N909AW, encountered moderate to severe turbulence while cruising at 35,000 feet over Alamosa, Colorado. America West Airlines, Inc., operated the airplane as a domestic, scheduled passenger flight under the provisions of 14 CFR Part 121. The airline transport pilot rated captain, co-pilot, 2 flight attendants, and 38 passengers were not injured; 1 flight attendant suffered minor injuries while another flight attendant sustained serious injuries. The flight departed Columbus, Ohio, about 2036 eastern standard time and landed at its intended destination, Las Vegas (LAS), Nevada, at 2227 Pacific standard time. Visual meteorological conditions prevailed and an IFR flight plan had been filed. Approximately 2 hours from landing, the flight crew called the flight attendants to inform them that turbulence was expected (based on ATC ride reports) and to secure the cabin and galleys. The flight crew said the airplane had been slowed to 0.78 Mach when turbulence was encountered. A flight attendant reported that the seatbelt sign did not illuminate immediately after announcement, so they continued service. About 10-15 minutes passed before the flight crew activated the seatbelt sign, at which point the flight attendants expedited their service tasks. (One flight attendant reported that before the seatbelt signs turned on, the turbulence had been "mild.") As the flight attendants were completing their tasks, the airplane encountered about 4 seconds of moderate to severe turbulence. The flight attendants took the most conveniently located passenger seats. One flight attendant, now in seat 34C, did not fasten her seatbelt immediately. She was lifted into the air during the encounter. She hit the overhead compartment and fell on the armrest and then to the floor. The first flight attendant briefed the captain on the situation and verified that the passengers were uninjured. The captain contacted the contracted medical service company (Med-Link); and a Public Address (PA) announcement was made asking for onboard medical assistance. A former Emergency Medical Technician (EMT) responded and assisted the injured flight attendant. The captain reported to ATC that the flight had encountered moderate wave with "moderate turbulence plus," and recommended alternate routing for other aircraft. The flight continued to LAS without further incident. The flight crew was drug tested (negative results), and the airplane received a structural inspection after the mishap. No structural damage was found.