Is it a conversion into negative, or a solarisation, or a negative enhanced? You know all those techniques, I know that, and try as I might, none of my experiments have so far come close to your results... Fill us in, will you?
Wow, we need another sub-forum. General is way too popular again. =)
LaFoto - this one was an enhanced negative. I used "New adjustment layer" and under the blend mode option I chose 'multiply'. (after a lot of goofing around with the different settings) After that I just had to add a lot of contrast since it was too dark.
Mess around with blend mode, you can get some really interesting results. Let me know if you hit a roadblock.