Last night I rooted my phone. If I want to return to an unrooted phone all I have to do is boot into SPRecovery and and pick the nandroid I created before rooting??
I am pretty sure there is more to unrooting than that, but you can search and find lots of info on unrooting... but you won't need to *wink wink*
With that said, if you need to go back to stock for repairs or just because, follow this link, it gives great step-by-step instructions for unrooting.