Here in Mississippi it involves 40 hours of classroom training by horticulture specialists with the state agricultural extension service. Classes are Tuesday and Thursday afternoons for 5 weeks and cover everything from basic botany, weeds, plant propagation, soil, and all sorts of plants information.
Classes are hooked up via satellite in multiple counties-we're about ten minutes from home. My partner and I are taking it together, along with 2 other men and 11 women in our location. Its all very laid back but highly informative and the first session flew by.
After completion, each master gardener owes 40 hours community service in any of a wide variety of programs for all age groups. Neither us us are partial to children so we indicated a preference for working with the elderly and they have a program that uses plant therapy with alzheimer's patients (don't really know anything else but its sounded like something we might really enjoy).
I've been gardening for twenty years but feel I've barely scratched the surface. I'm really psyched up about this. We're going to have a great time, meet some wonderful people and learn some valuable information. And I'll finally "master" something.