Artist Phil Caesar posted a whole pile of backgrounds up on his blog almost a year ago. They appear to be as far back as November 2010 in post date.
The cool part? Some are completely un-aired Canterlot images, similar in art style to Trixie's image from a while back. I don't know if these are from Season three, or were a part of an episode that just never went up, but they are definitely exploring a part of Canterlot that we have yet to see.
I'm sure you all all agree once you see them, that these need an episode! Head on down past the break to check both out and avoid any spoilers.
Thanks to Ace Reporter Whatshisgame for the heads up!
Note: Some are confused on where Canterlot came from. If you visit the actual blog post, the black and white versions read: "Canterlot Alley" and "Curio2"