As reported by Ryan Healy in the Diaries, Gil Meche has strained his oblique and will be out of action indefinitely.
Mariner fans probably remember when Rafael Soriano came down with the same problem back in the spring of 2004; after straining his oblique in late February, Soriano didn't appear in a competitive ballgame for more than a month, and the injury continued to bother him into early May (at which point he blew out his elbow). It's a nasty injury. Not only are obliques notoriously slow to heal, but they also have a tendency to linger and recur down the road, all but necessitating a contingency plan. This probably isn't how Gil wanted to kick off his contract year, although given his medical history, it's fairly appropriate.
Without any more information, the best we can do is make guesses about his timetable. If it's a minor strain, Gil should be out for about 1.5-2 weeks. A little worse and we're looking at a month. And if there's tearing - he hasn't yet undergone an MRI exam - then it'll be worse still, probably sidelining him for at least two months, and almost certainly more since he'd have to build up his arm strength before returning to action. We'll know more in a few days.
In the meantime, while I'm a little disappointed that Meche has hit a roadblock on the way to becoming a two-seaming dynamo, this opens the door for Clint Nageotte and Jesse Foppert to show what they can do. While neither is ready to be handed a Major League rotation spot out of the gate, the organization may not have much of a choice, depending on the severity of Gil's injury. Kevin Appier and Dave Burba have pretty much already played themselves out of consideration, which limits the options. I suppose Jeff Harris is floating around, too, but he needs to get himself back on the roster before he thinks about pitching in Seattle again.