I did the research about a year ago as a room parent, and there were no guaranteed peanut-free bakeries that make fresh baked goods that I could find. It's not that you can't get cakes that don't have peanuts, it's that they aren't from a peanut-free bakery so kids with the severest allergies have to avoid them in case of cross-contamination.
But wow, imagine the market, especially if they could make lower-sugar, trans-fat free bakery items for class parties!
We buy Enjoy Life brownies (in a box, at Whole Foods) for class parties now. Enjoy Life is also local (schiller park, I think), so I like to support their mission - their products are free of all of the most common allergens. They aren't cupcakes, but they taste pretty good. My non-allergic kids love them. You might also check with the mom - we know a gluten-sensitive child who is invited to daycare parties with my son and his mom always just brings a cupcake.