Ladd, The latter. You can live in Montana for years and still not know all the climbing areas. But look at it this way--a lack of knowledge about a route maximizes your sense of adventure. A first ascent is only a state of mind. That said, plenty of MT guidebooks exist for those who want to maximize their route count.
Yes access issues. The montana climbing moffia kept Lost Horse secret, even from the forest service. Now it's slated to be quarried into chip seal. http://firstascentpress.com/losthorse.html It's not so secret now!
The climbing scene may be under the radar, but you just have to make friends with a couple of climbers (by supplying them with copious amounts of beer and meat, preferably Elk) and you'll easily get all the info you need.
It is really strange how Montanans want to keep areas a secret. I lived in Bozeman for ten years and climb about everywhere in the guide books and other areas not in guide books. Compared to other destination areas in the USA I do not see the big deal. I have never found one specfic area in Montana that I would consider a "Destination Climbing area" such as Red Rocks, City of Rocks, the Sierras or the Rockies. Just a smattering of crags here and there with lots of road between them. There is climbing south of great falls in White Sulfur Springs. A hand full of routes on the Stone Temples. Hard to find, hard to get too unless you know the area or Ron Brunkhorst. However I would take Jons advice. Get a guide book and go if you are so inclined. However I live in Fort Collins and I would take Lumpy Ridge Climbing over anything in Montana. Super solid Granite, hundreds of routes, up to six pitches, beautiful abundant long splitters and very few people unless you are doing a hand full of classics. People do not seem to like climbing without bolts nor bolt stations that much. Or go do a wall in the backcountry of Rocky Mountain National Park. Some areas you would be lucky to find another party.