The sky is frequently a bit of a problem for photographers. Usually the sky is so much brighter than the ground below that it tends to wash out in the final photograph.
If you try to restrict the photo to the sky alone you lose a sense of scale. And those clouds that look so visually appealing to the eye may fade away to pale smudges against against a pastel icy-blue sky.
Although the iPhone camera is not especially well designed for adding physical filters to the lens, I've found that you can simply take a photo through one lens of a pair of 'polaroid' sunglasses to get a very decent pola-screen effect.
Fortunately if you don't have your shades with you, Iris Photo Suite now includes a polarizer filter among its photo fx. This nifty effect can turn your bland sky into a visual delight - quite the sort of thing you originally imagined when seeing those magnificent clouds.
Lots of other photo processing goodies in Iris Photo Suite as well. This may just turn out to be the Photoshop of the iPhone.