Definitely go! The parks are generally less crowded when it rains so that can make life a lot easier in the long run.
Also, I agree with the PP. The rain is usually showers that do not last all day long. We got back on 2/10 and the day we planned to go 2/3 the forecast was 50 % chance of showers and overcast all day. Well, I was psyched and the crowds were non-existant. We literally waited for one ride (Magic Carpets) for one cycle of the ride. Boats were going out empty on Splash Mountain! By 11:00 the sun was shining and it was a beautiful day. It never did rain, although it was foggy in the morning. I don't trust forecasts that far out anyway.
Also, buy the bright blue ponchos from Walgreens so you won't be lost in a sea of Walt Disney World yellow ponchos.
Have fun!