I'm really impressed with the quality of the electric guitar libraries that have come out in the last couple of months. It seems to me that now the accoustic guitar market is open to similar improvement. The only accoustic of consequence that I'm aware of is the RealGuitar. It's now long in the tooth and I believe it uses 16 bit samples. At any rate even though I have the Real Strat, the Real Guitar has never sounded convincing to me.
On a different topic. I know of no pedal steel libraries out there. A developer could really differentate themselves by putting out a high quality pedal steel library.