Piston slap is basically where the piston skirts are short enough to allow the piston to "wiggle" side to side while traveling up and down in the bore. The piston has to be slightly smaller than the bore, and the rings take up the slack. If the piston is short and the wrist pin is roughly centered vertically in the piston, the piston can "wiggle" as it travels. Toyota actually released a TSB for this concern back in 1988, and redesigned the pistons to reduce the problem. All engines manufactured after 1988 have the new design pistons in them, and it shouldn't be of any concern. Also, any pistons purchased from Toyota since then should be the new design.
I have the TSB on my toolbox at work, as I'm rebuilding a spare 22RE right now. I'll try to remember to bring it home and scan it if anyone wants to see it...
Ryan
