Of course - while the code was easy - making it look hot was more work. Luckily, I was contacted by Amin Eftegarie. Amin graciously offered to do the design, and pushed me to actually put this code up on a domain and share it with the world.
You can run this code yourself at popularfollowers.raymondcamden.com. All the design is credit him. Trust me - my initial version was slightly uglier. ;)
Unfortunately, this solution is probably not as "tight" as I would like. I was never able to properly handle the error case for when a username didn't exist. I can handle errors in general, but I couldn't specifically notice/handle that case. Also - it's pretty easy to hit your personal API limit. I'll consider adding an OAuth type setup... but frankly... it works "good enough" for a toy and I'm satisfied with it as is. It also takes a little while to run. I've got 4.4K or so followers and it takes close to a minute to get them all and sort them out.
Oh - and I discovered that Melody Thomas Scott follows me. Awesome.