I have heard that APs with a diversity antenna configuration make sense for 802.11b but that with OFDM coding two antennas don't give much benefit in 802.11g.
I'm not really sure if the effect, or lack thereof, is different with 802.11 b & g.
From what I understand, antenna diversity is when an AP has multiple antennas to "sample" the signal at different locations, in order to choose the best one. This is used to mitigate the effects of multipath.
Hope this has helped. I'm sure someone will be able to give you a better answer though.