UVMapper is a great tool! The Pro version is something we use on a day to day at Smith Micro.  It's saved my butt many a time. :) 

I have not tried exporting and then re-importing with Sculptris yet. I only started using it this afternoon (I did give it a brief look before 1.0).  When I first saw the app, I scoffed at it. However, after actually trying to make something in it today, I found it has a lot of power.  It took me a minute to realize that I should hit cancel when I import a UV mapped OBJ if I want to sculpt on it.  Sounds like UV data may get thrown out if you're just sculpting, though I don't know for sure. If so, that's a really odd way of handling importing and may have something to do with the problem on export. 

For a free app though, I'm not really complaining. I now have something to keep my nephew busy whenever he sees me using ZBrush and wants to do the same thing.