According to Colin Dunlap and Jenifer Langosch, the Pirates have claimed lefthanded pitcher Garrett Olson from the Seattle Mariners. There will be a corresponding roster move coming. Olson is 27 and has been in the bigs for four years. He converted from a starter to a reliever during the 2009 season. Last year he appeared in 35 games and pitched 37 innings.
The short answer is he has not had success in the major leagues. The Pirates are obviously looking for lefthanded help in the pen, but I wouldn't put it higher than 50% that Olson goes north with the team.
Update: Kevin Hart has been placed on the 60-day DL, clearing a spot on the 40-man roster for Olson.
Update 2 (by Vlad): A little more background on our newest acquisition.
Olson was at one time a fairly well-regarded prospect. The Orioles made him a sandwich pick in the 2005 draft, out of Cal Poly San Luis Obispo, and he signed for a $650k bonus. Olson earned a spot on BA's list of Baltimore's top 10 prospects in both 2006 and 2007. He was also honored as the Orioles' Minor League Player of the Year in 2006, and was chosen to pitch in the Futures Game in 2007. As noted above, however, he failed to establish himself in the bigs as a starter, and he was traded twice in the 2008-2009 offseason: First from the Orioles to the Cubs as part of a package for Felix Pie, and then from the Cubs to the Mariners less than two weeks later (along with Ronny Cedeno!) in exchange for Aaron Heilman. The Mariners converted him to a situational relief role last year, and he performed acceptably for them, enjoying his first taste of major league success.
Olson's fastball sits in the upper 80s and can touch 90. His best pitch is a big curve, on which he relies fairly heavily. He also throws a change. He has flyball tendencies, and while he probably has enough stamina to go more than one inning, his problems against right-handed batters would seem to make that a bad idea. I believe that Olson has a somewhat better shot at the Opening Day roster than dtoddwin does, if only because he's out of options, and thus would need to slip through waivers in order to be retained.
Olson's 2011 ZiPS projection with the Mariners calls for a 4.75 ERA (86 ERA+), 1.49 WHIP, and 81/52 K/BB in 110 innings pitched. That projection reflects Seattle's park and league context, however, and would likely shift slightly when placed within ours.