Yeah I'm running 700x35 Bon Jon Pass tires and clearance is comfortable. You
could probably squeeze a 700x38 Barlow Pass in there, but to Bill's point, the clearance would be pretty tight. It depends on brake choice, too- I have a Tektro 539 in the front that I don't think would accept a 700x38 tire, but the Shimano BR-something or other on the rear likely would.
Erik, Philly