Wet & Messy fetishists are more common than I had originally thought. When I was first starting out and was just doing fetish modeling, I'd have to say Wet & Messy photo sets were definitely in the top 5 requests.
http://umd.net/
http://www.messyfun.com/
I know a girl in Tennessee who makes a pretty good living off her Wet & Messy site alone. Whipped cream, mashed potatoes, chocolate syrup, melted cheese, mud, pudding are all popular substances to smear all over yourself.
Personally, I don't like anything in my hair, so that's out for me lol.
I think part of the appeal (for the WAM photo lovers anyway) is to draw another couple of senses into the photo. They might not necessarily know what the woman in the picture smells or tastes like, but they know what, say creamed corn or mud or lime Jell-o tastes like, so they can insert that into their fantasy.
That's just my guess though.
I am quite intrigued by the whole thing... (well, I find most "unusual" fetishes intriguing haha).