On May 23, 2011 at about 1520 eastern daylight time, a Boeing 757-223, N623DL, operated by Delta Airlines as flight 2001, encountered turbulence while in cruise flight from Boston Logan International Airport (BOS), Boston, Massachusetts to Atlanta Hartsfield International Airport(ATL), Atlanta, Georgia. The flight was operated under the provisions of 14 Code of Federal Regulations Part 121 as a regularly scheduled passenger flight. One passenger was seriously injured and one of the 4 flight attendants experienced a minor injury. None of the 2 flight crew, other 3 flight attendants, or other 180 passengers was injured. The airplane was not damaged, and continued the flight to Atlanta. The flight crew reported that while in cruise flight at FL340, approximately 200 miles northeast of the destination, they were in an area of forecast convective weather and were maneuvering between two cells when they inadvertantly entered the edge of a cumulus buildup at the time of the turbulence. The seat belt sign was illuminated at this time, but the passenger was out of her seat and fell, resulting in a broken ankle. Probable Cause: Inadvertant encounter with convective turbulence while in cruise. Contributing to the severity of the accident was the unrestrained status of the passenger, contrary to the lighted seat belt sign.