I am always interested in small bikes so your question interested me. I did find one single speed full suspension "mountain" bike: http://www.lenzsport.com/detail.php?prodID=4
But, I don't know if the smallest size is small enough. After all, it is a 29'er.

I think your idea of an urban/city bike with a suspended fork makes sense, and then either add a sprung seat or seatpost.