There is no problem with early spaying, as Spencer noted. Some organizations will spay at three months. I think the few organizations that neuter that early are taking all things into consideration - the problem with getting adopters to bring their cats back in for neutering, even if the fee is paid in advance. I think they figure the increased risk of FUS is counteracted by the decrease in overpopulation. And, like Troy mentioned, there are many things that are suspected of increasing the incidence of FUS, but all are only suspected. There is no definitive reason why some cats succumb, and others don't. My cat was almost a year old when he was neutered and never ate dry or high ash food, but still suffered from bouts of FUS.

On the other hand, my current boy was neutered at 4 months, but I was afraid to wait any longer as he was already 8 lbs and had two littermate sisters. He is now about a year and a half old, but so far has never shown a sign of FUS (although I'm very aware it could still crop up).

So, like alot of veterinary decisions, you have to take everything into account. My somewhat educated opinion is, if there is no reason not to, then wait 'til your boy starts getting frisky. (Girls, however, should be spayed before their first season, if possible, to avoid a greater risk of breast cancer.) Otherwise, do what needs to be done in your situation.