Chad, I misread your question before. I thought you were wanting to swap hubs on a wheel - to swap it from a multi speed hub to a single speed coaster brake hub.
I see now that you are just talking about getting rid of the cassette or rear gears on a multi speed and going with a single speed gear instead.
Steve said he tried that once and he just took his rear wheel to the LBS and they pulled off the cassette for free and then sold him the single speed cog for something like $12. The problem he ran into was that the single gear didn't line up with his front sprocket and so the chain kept jumping off.
And I know from reading a few things that in some cases you can just get a single speed cog and screw that one on where the cassette screwed off, but on other multi speed wheels where the gears don't screw off (not a cassette - but a freewheel type) you have to use a single rear gear and some spacers to fill up the place where all of the other gears used to be - and to also allow you to put the one gear in alignment with the front sprocket.
So your best bet may be to take your wheel down to the LBS, tell them what you want to do on it, and then let them tell you what's involved.