This was photographed just outside Crested Butte, Colorado and was photographed in the summer of 2012. It is a 3-photo HDR image that was processed using both Nik's HDR Efex Pro 2 software as well as Photomatix Pro.
The rainbow that appeared after a brief afternoon shower was one of the brightest rainbows I had seen in some time. I quickly grabbed my camera from my RV, set it up on my tripod, and started firing away.
You can read a full behind-the-scenes account of this photo complete with before photos here