This is an old bug, from VFP5 and 6; Q190267 was the KB number assigned to it.
I somehow thought it was fixed - but it wasn't. I have a bunch of combos bound to two column arrays where some of the values are negative or zero (bound column is 2, column 1 contains descriptions, .bound=.t.). No matter what I do, the displayvalue remains blank for .value<=0... which isn't exactly helping.
I'm writing this just to confirm that the bug is still here, alive and kicking. Since nobody seems to have complained about it since 2002, my guess is it won't make it into SP2.