Bly originally played for the Lions from 2003-06 and made the Pro Bowl twice during his stay in Detroit. In 2007, Bly was traded to the Denver Broncos in a deal that sent Tatum Bell, George Foster, and a fifth-round pick to the Lions. Looking back, it was a pretty bad trade for the Lions, which is no surprise for any move made by Matt Millen.
The Broncos released Bly in February 2009, and he went on to play for the 49ers last season, picking off three passes in 16 games for San Francisco. From 2006-08, Bly started every single game, but he was only in the starting lineup for six games in 2009.
To make room on the roster for Bly, the Lions released cornerback Paul Pratt.
UPDATE: Here are some comments from Bly on returning to Detroit:
“I feel like I played my best ball here in Detroit,” said Bly. “I feel like it’s home and to come back and have the chance to finish my career where I played my best ball – where I feel like I’m part of the community – is a great feeling.” [...]
“I’ve also heard the attitude of the team is completely different. I felt like I was a part of the community when I was there in Detroit. I always felt like I was family. I had a great relationship with the people upstairs, the administrators, people that worked around the facility.”
“I feel that when the Lions have struggled over the years, I’ve struggled as well because, like I said, I feel like I’m a family member,” he said. “I’m glad to be back. Hopefully we can get things turned around and I just want to be a part of the puzzle.”
Bly also mentioned how he has heard great things about the likes of Ndamukong Suh and Louis Delmas, who are now his teammates and fellow members of the Lions' defense.