From Pro Football Talk:
Amid rumors that the Detroit Lions are involved in significant trade talks, a league source tells us that the subject of the discussions is defensive tackle Cory Redding, and that the team to which he might be traded is the Seahawks.
The post goes on to mention that Seahawks linebacker Julian Peterson is going to be released soon, so maybe he could be involved in a possible trade. He and Redding's salaries are both large, so that would make sense for both teams cap-wise. My only other guess is that the Lions could acquire Seattle's #4 overall pick as well as a couple other selections in exchange for Redding and the top overall pick. That way the Lions could move down and still take someone they want for less money and get extra picks as well.
UPDATE: Tom Kowalski reports that the rumor has been confirmed and a trade would involve Julian Peterson.
UPDATE II: PFT is reporting that the Lions would receive a pick if the trade happens.