Laurie Ford offers some thoughts on sails (and many other subjects!) on his site - some good pics of kayaks under sail in biiiiiig seas.
From Tasmania, the Maatsuyker Canoe Club has some pics of kayaks with sails and some stunning paddling pics.
Again from Maatsuyker, this article has some designs.
"Gnarlydog" offers some thoughts on DIY kayak sails in his blog - well worth looking at as it also includes a mass of detail on building and mounting the mast.
Probably from the States, this has got to be the ultimate sailing sea-kayak - try capsizing this one.