How many people are still rocking original iPhones?

With Apple officially discontinuing support for my beloved 2G, I’m curious how many of these “antiques” are still in use. Even better would be a percentage breakdown for 2G, 3G, and 3GS.

I’m guessing the most authoritative source would be browser stats from somewhere like ComScore. Total sales figures would be interesting too, but of course they’d grossly overestimate the number of 2Gs in active use.

I’m especially curious about this because I’m starting to learn programming for the iPhone, and with all the attention focused on the iPad and the latest iPhones, I feel like there might be a real opportunity in catering to plucky old schoolers like me.

Thanks in advance!