He got released by the Blue Jays after they activated their closer Scott Downs from the DL. He's left-handed, which the Cubs could use in the bullpen. His numbers haven't been good (6.53 ERA, a 13/17 K/BB ratio, and a 1.89 WHIP), but his velocity being down has to factor into that. I'm guessing that they'd only have to pay him the minimum if they sign him, and w/ Dempster on the DL, they could use Marshall in the starting rotation and use Ryan as the lefty specialist.
He's a former closer, so if Gregg or Marmol needs to rest, he could close a game or two. There's not too much downside to signing BJ Ryan.