Clicky:
http://www.rv.net/forum/index.cfm/fuseaction/search/parms/sw%7Bcork%20flooring%7D%7Ckm%7Bexact%7D%7Ckl%7Bm%7D%7Cfm%7B%7D%7Cpd%7B365%7D%7Cma%7B%7D/sr/1.cfm Need for an underlay depends on brand/construction of the product.
With the U.S. Floor product we used, there was a "built-in" underlay-- no additional underlay was recommended.
It is a floating floor, as virtually all natural products are/should be, as there is a small amount of movement with changes in temperature and humidity.
We just left a 1/4" gap at all walls and installed quarter round glued to the wall to cover it. After a season with it, we are very happy with it.