There is a restaurant called Hugo's Inn on Route 1 in Kennett Square, PA, that has a huge selection of mushroom dishes, including a small cast iron skillet filled with quartered sauteed mushrooms and a Philly cheese steak with peppers, onions, and melted provolone but with mushrooms instead of steak. You wouldn't think to stop there, it's just a big old whitewashed building. Inside is a knotty pine panelled big barroom with a little dining room in the back.
About 3 miles down the same road is the Phillips Mushroom Museum, where you can buy wooden baskets of just-harvested portobellas and other varieties at wholesale prices.
Edited to add that Hugo's closed its door in summer of 2006. Here's an article about their demise. Just wanted to update this thread so no one would try to go there and be disappointed.
You could combine the two into one mushroom roadtrip.