how can i seed 0.5 cell /well? - (Jul/16/2008 )

This may be late but helpful for future reference. To isolate a single cell, I just plated a small amount of cells in one well (50-100 cells) and did a 2-fold dilution series over the rest of the wells. The real trick is to keep your cells from clumping and shearing them by pipetting.

PS You should probably not vortex live cells.