So I looked them up, curiosity raised. They've been around for a few years now, these "fatties" and are now filtering down into the mainstream where us bicycle trend clueless people live. These tires are good for your butt over long distances as they are cushy in their girth. They also go like gangbusters in snow and sand (who knew?). Apparently, they shouldn't replace your regular bike according to one expert I read ... but they are a lot of fun to have in the stable.
Gotta say, on second glance, they do look like fun.
And that's the five minute response.
For more, see the people for bikes blog article: http://www.peopleforbikes.org/blog/entry/why-fat-bikes-could-be-for-you