I'm not 100% sure if this is actually Jon Jansen or not, but a Twitter account that supposedly belongs to him posted the following tweet this morning:
I was just told by the Lions I'm being cut today. Needless to say I'm disappointed. I enjoyed playing for my home team and the city of Det.
If true, the Lions have decided to keep the younger Corey Hilliard instead of the veteran Jansen. Hilliard was given a big vote of confidence when he started at left tackle against the Bills, and with room for only four tackles, Detroit simply didn't have room for Jansen with Hilliard on board.
I personally believe it would be wise to keep Jansen around, but one of the things with roster cuts is that tough decisions will be made, and this was definitely one of them.
UPDATE: All signs point to that indeed being Jansen's account, so I think we can safely say that the Lions have released him.
UPDATE II: Dave Birkett talked to Jansen and got some interesting quotes from him.