For the type of fading that 802.11b is trying to combat, the signals from two antennas separated by even a wavelength are largely uncorrelated. Hence, even very small separations work.
The spacing is non-critical. Since the receiver is only using one antenna at a given time, you don't have to worry about the phase relationship between the two antennas. If you were using two antennas at once (typical in two-way radio and broadcast TV/FM, etc.), the spacing and length of feedlines is critical because there the phase matters.
