I haven't had the best of luck capturing rainbows, espcially since they are so fleeting and require you to be at the right place at the right time. Yesterday, one of my co-workers alerted me to a rather colorful rainbow that had formed with the mix of sun and rain we were experiencing. I quickly grabbed the D40 and attached the 70-300mm VR, and went crazy. I was happy to get a few workable files and applied by "Velvia" work flow to enhance them.
This is yet another example of how important it is to have a camera with you at all times.