This is it - No Photoshop! I could see the end of the rainbow intersect the ground as I was driving to a good spot to shoot it. I ended up at Hopkins point in Mendon Ponds Park. I could see both ends hitting the pond and a full arc across the sky. Unfortunately, I was too damn close to it to get a good shot of the whole thing. I did a composite photo here - http://www.pbase.com/timbg/image/42701614
When a rainbow intersects the ground, you see a spot of very bright golden light - I beleive that is where the "pot of gold" story comes from.