Washburn has an 8-6 record and 2.64 ERA in 20 starts with the Mariners this season. He's lost only twice in his last 10 starts, and was arguably the third-best starting pitcher available on the trade market after Roy Halladay and Cliff Lee.
No word on who the Tigers may have given up yet. This post is probably going to be updated.
UPDATE: Pitchers Luke French and Mauricio Robles are heading to Seattle.
Robles is a left-hander who's pitched in 18 games this season between Single-A Lakeland and West Michigan, compiling an 8-6 record and 4.24 ERA. The number that raises an eyebrow (and seems to have Mariners fans excited) is the 111 strikeouts he's thrown in 91.1 innings.