Because of a trade made during the 2010 NFL Draft, the Lions actually had an extra sixth-rounder. The Eagles made a move to get one of the Lions' seventh-rounders (the 220th pick to be exact), and as a result the Lions got a sixth-rounder in 2011.
Other moves involving picks in the 2011 NFL Draft include the Kevin O'Connell, Chris Houston and Shaun Hill trades. The Lions got a seventh-rounder from the Jets for O'Connell and sent a seventh-rounder to San Francisco for Hill. The Houston trade resulted in a conditional seventh-round pick being sent to Atlanta, so it's not guaranteed that the Lions will lose the pick.