http://www.nfl.com/news/story/0ap300000 ... retirement
This has been a big talking point in these parts over the past week. Speculation is that Calvin is leaning towards retirement. You can read the article, but the speculation comes from some of the answers he gave after week 17 and some discussion from people in the know. Nothing is official, but even the Lions have made comments that seem to think he is considering his future.
If this happens it would be the 2nd time that one of the best players in the NFL has left the Lions early. Barry Sanders did the same thing at age 31. The main difference is that Barry left because he couldn't take the losing culture in Detroit and Calvin is leaving because he wants to get out while he is relatively healthy.
I would actually have no issue with this. This would save the Lions around $20 in cap space and allow them to pick up a top level free agent.