Both my mother and I have the white models of this hat
http://www.outdoorresearch.com/en/wo...-sombrero.html
It comes in 4 sizes and the crown of the hat is soft so it packs down well in a "flat" position. I just put it in my carry-on last and clip the compression straps over it. Brim is made of a pliable foam so it straightens out just fine. Quite light, too.
Eric Javits makes a cute rollable "Squishee" hat. I've also heard great things about Tilley.