Tom Coates has written a surprisingly interesting piece (design-wise and meaning-wise) about the US states flag.
Similarly (is it the flag season?) Gail Orenstein, a flickr contact has recently posted an article about the origin of the British flag (Union Jack).
Finally, here is a link to aphotograph of the Catalan and spanish flags. You'll notice (although it's not easy to spot on the photo) that part of the catalan flag is identical to England's flag.
The reason is that both Catalunya and England share the same Saint protector: Saint George, symbolised by the red cross on white background.