I don't know about 20 years, but they have certainly been talking about it 10 years ago. They got real quiet about 4-5 years ago and won't discuss it; some say that means they dropped it, some say it means they are working on it. I say it means they learned their lessons about talking about products in development prior to release (just look at the current Bonanza autopilot fiasco) and they have plugged up their internal information leaks.
It's a big hill to climb, I certainly hope they are attempting it.
I sure hope they don’t give up on a Navigator. Their stuff integrates nicely and if their Skyviews can be used for display/control they will have a large client base of existing and new customers. Fingers crossed.