[MrSteve]

Its a bit fiddley but there is a way to do it, because you can toggle 1 sided shapes and 2 sided shapes, and 1 sided and 2 sided surfaces, so depending on how desperate you are you can select the surface(s) and hit the approriate "1S" in set surface type and "Flip Normal" in the Surface menu! see what you think.

SIDE NOTE: If you are doing this on a large model or finished object I would NOT suggest making the whole object 1 sided because you could find that the prime normals are facing the wrong way and you will have 4 extras hours work to do!

Hope This Helps!