volts500 is on the money. I also beleive (but am not completely sure if
this is actually in the NEC) you are supposed to label all of the
ungrounded-but-gfci 3-prong outlets with a sticker saying "NO GROUND"
or similar. If I remember from last time I did a gfci, the box even
included a half dozen such little stickers. Or maybe that was just the
"GFCI" stickers I'm thinking of... hmmm.

-Kevin