This comes from ESPN's Adam Schefter via the Twitter:
With all of the focus from the past couple of days having been on Braylon Edwards and James Jones, this signing pretty much comes from out of nowhere.
Jenkins, who turned 29 back on June 18, was the Falcons' first-round draft choice (29th overall) in the 2004 NFL Draft out of THE. . .dramatic pause. . .Ohio State University. His best season in Atlanta in terms of yardage came in 2008, when he racked up 777 receiving yards on 50 receptions. For his career, he has 276 catches for 3,512 yards and 20 touchdowns.
Jenkins is a bigger receiver, measuring in at 6'4" and about 215 pounds, but he's never really been anything more than a second or third option for the Falcons. Still, he has the ability to start on the outside in this offense, which is more than many of the receivers this team has right now.