I've updated my MiKTeX installation, between the packages that need update there were pgf and pgfplots, and they're used by tikz. Now your example doesn't work neither for me, I think one of those two packages broke something with the preview-latex package, since they still work with ps2pdf. pstricks is still working, as it doesn't use pgf. What to do then? Maybe we should contact TikZ or maybe the preview-latex ones.
I wanted this feature so much that I worked it out, and now they broke the compatibility with preview-latex! venom00
