I'm with maggie - its called a platform valence and goes UNDER the mattress, a valence sheet goes OVER the mattress and acts as a bottom sheet to lie on.
The link above gives pages and pages of links to base valances 4get. I wish I could use one on my bed but because it is an adjustable one you can't put anything on the base that would interfere with the gubbins that controls it :(
i think you can get elasticated ones that are basically a big strip of ruffled fabric with elastic at the top... i don't know what they're called though, sorry.