After a fruitless search for a free, Creative Commons-licensed vector illustration of a cupcake, I decided to pull out Illustrator and make one myself. If you wish to use this for your own projects or site, just download it below. All I ask is that if you do use it, you won't use it for commercial purposes without asking and you provide a link back to Nested.
Cupcake vector by Mena Trott is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 United States License. Based on a work at nested.typepad.com.