Lori Blumhagen sent us this photo of a rainbow around the sun while she was down in Key West, Florida a few weeks ago. (The dark gray triangles are sails from the boat she was on.) She says she and the locals had never seen anything like it before and wondered what it was.
It's actually a rainbow's twin brother, in a sense. What you are seeing are tiny ice crystals in the thin overcast present there reflecting the sun's light like a prism -- much in the same way raindrops reflect sunlight to make a rainbow. Here the halo is seen at the exact angle needed to make the prism effect -- 22 degrees. There is more info and the physics behind them at this link
They are seen worldwide -- we occasionally get them here in the Pacific Northwest as well. I even have a pseudo-gallery of such events in our Weather FAQ.
And it's not just for sunshine. Halos can form around a bright moon as well.
The halos have some forecasting lore around here in that they typically mean steady rain is coming within 24 hours or so. That's because those high, thin cirrus clouds that are formed by those ice crystals are usually the first fringe of an approaching cold front.
By the way, sometimes cirrus clouds at just the right angle from the sun can make a rainbow effect as well. I had heard them called "Mare's Tails" but I don't think that is an official term.
That doesn't necessarily mean rain soon, just that you should have had your camera ready :)