I googled a bit and found several sites that talk about litter box mold. None were good enough to link to, but in essence it boiled down to a few factors.
Warm, moist environments are very conducive to mold growth, so moving the box away from the warmest part of the room may help considerably.
Some litters tend to grow more mold than others (organic litters like those that are made of corn or paper may grow mold faster than clay litters).
Changing the litter/cleaning the box more frequently can also help.
Last edited by kaplods; 03-25-2009 at 08:00 AM.