He might even start tonight, but for now we just know that he's at least getting called up. Rumor has it that Tim Lincecum has a sore back, and that's why Bumgarner is getting the call. I don't like that rumor, though, so I'm just going to ignore it.
But you're welcome to use this thread for an old-fashioned "Lincecum is hurt and Bumgarner's arbitration clock is starting and what is wrong with this organization and why am I bleeding from the eyes and they released Travis Denker what in the hell" freakout thread.
Edit: Update from Schulman:
I want to stress that I have not confirmed this directly, but team officials are not denying The Chronicle's suggestion that Madison Bumgarner is being promoted so he can start tonight's game in favor of Tim Lincecum, who cannot start because he is hurt
Edit: Update from the press conference: It's a back injury. Lincecum is day-to-day. Unless he's not injured, like some callers on KNBR think. Conspiracy! Or, maybe his spine melted in a freak industrial accident and they're just not sure how to tell us. I recommend that you all continue to freak out.