The Phallic wonder strikes? Seriously though, no one really rides proper fixed MTB's on real trails. There's a lot of skill while trail riding in timing your pedal strokes to make it through rock gardens which requires a freewheel. As for single-speed mountain bikes.... There's quite a few frames that have replaceable drop-outs to go SS.
As for converting your current bike, you can pick up a White Industries ENO hub and build a rear wheel for that, and you're good to go. There's also a variety of conversion kits available which replace your rear cassette with one cog and some spacers to to set the chainline straight.