It takes more than just being found with several pounds of weed for the Bengals to keep you out of action. From ESPN's Chris Mortensen: 

To be fair, Simpson hasn't been convicted of anything, even if the evidence seems overwhelming. And as Mike Florio points out at Pro Football Talk, the NFL, not the Bengals, is ultimately responsible for deciding if Simpson will be suspended. Despite the potential distraction that his presence with the team may cause, it won't be a surprise if he evades any punishment by the league until his legal situation takes its course.

The Bengals haven't officially confirmed Simpson's status for the game, but word will come soon. From the team's official web site:

The Bengals wouldn't confirm an ESPN report Sunday morning that wide receiver Jerome Simpson is playing against the 49ers in Sunday's 1 p.m. game against the 49ers at Paul Brown Stadium. They are expected to announce his status before the inactve list comes down at 11:30 a.m.

It's believed that Simpson was at PBS Saturday after missing practices Thursday and Friday following reports that he's under investigation in a drug investigation.