OK, I'll throw out the ugly dose of reality.
We visited the last week of September '02, and it rained the first four days of our trip. And when I say it rained, I mean it RAINED. It is hurricane season (Isabel, duh!), and sure enough a couple days before we arrived one had swept into the gulf--sorry don't remember the name. By the time we hit Orlando, the storm system had moved east and right into town with us.
We had rain at least a half-day for those four days straight.
Best advice--be prepared for it. Bring ponchos and umbrellas. Make sure everyone in your party is prepared for the rain so that tempers don't flare. Think about things you can do indoors if you just can't take anymore rain. Find out where the laundry facilties are in your resort (assuming you aren't staying in a home away from home).
On the brighter side, rain has a tendency to empty out the parks. If you can weather the storm (literally!), you may be nicely rewarded if the rain lets up after a short period. One of our fondest memories from last fall was a nearly empty Tapestry of Dreams parade at which our son got unlimited attention from the performers because there was hardly anyone at Epcot.
Hope for the best, but prepare for the worst. At least you won't be caught off-guard.
Hope you all have bright and sunny days waiting for you...