I just replaced the lower intake manifold gaskets on a '99 Olds 88 after noticing what appeared to be a coolant leak. The upper was done at a service shop about a year and a half ago, but they did not do the lower. The car only has 48,500 miles on it, and is not driven hard, as it is owned by a 90 year old woman. She doesn't drive like your typical "old lady", however. (Not exactly the "Little Old Lady from Pasedena" as mentioned in Jan and Deans song, but she holds her own with traffic.) She has regular maintenance performed, and had the DexCool flushed out after 3 years. I was surpised at how bad the gaskets looked.
The heads and LIM showed some signs of pitting, also. (Sorry, no pics of that as I didn't have my camera with me.) There has been some discussion on when to expect the lowers to go, so I thought some may be interested in seeing these at this "low" of mileage.
The one thing regarding her driving that is likely a factor is that a lot of short trips are made with the car. She lives in Detroit, and often just drives to a nearby coffee shop, to church, and to the store. Her last car (I think it was a 96 eighty-eight) spun a bearing after the gaskets were replaced. This could have been a dealer service problem, as GM ended paying all but $50 to have a short block put in.
It would seem that how the car is driven during the miles that are put on is at least as important as to how many miles are on the odometer.