+1 ^ MDF will give you a very smooth floor surface and is dense enough to hold up to a lot of abuse. I have also used 3/8 smooth finished plywood over an old cotton warehouse floor that wasn't in the best condition and it worked out quite well.
Here's a shot that I had from our last studio. I'll see if I can dig up some more later.
Ryan Klos (ryanklos.com) has written a book on backdrops, infinity coves and cyc walls. It should be coming out soon and should have some good stuff in it as well.
Even if you don't get it perfect, remember that with just a little more light or some quick work in Aperture / Lightroom, and it'll look just fine! Just don't freak out the first time someone leaves a bunch of marks and scratches on it
, as it will happen sooner than you expect it to!
You might have success with Rosco's Colorcoat or a two part epoxy based floor paint, which are a bit more durable and require less frequent touch-ups than white deck paint.
Good luck with your project!