Luckily trail access here in Maryland is awesome. It is very RARE to see any trail closed. On the other hand, I just got back from the Finger Lakes region of Western New York and access is terrible. The rule there is no bikes unless explicitly posted as being allowed. I had to travel an hour and a half to the nearest trail. Not a good situation up there.
You're right...I didn't even notice that you couldn't grade outside of the states. I guess that's not very international of IMBA.
Downieville and Tahoe are beautiful places to ride. The Bay Area has some excellent riding but there is a lot of trail conflict. With so many people living here and with the sunny weather everybody wants to be outdoors.
I've heard and read about all of the great riding in Colorado. Going to Colorado to do some mountain biking is definitely on my "to do" list. I'd like to take a couple of weeks and explore the state. Plus give myself some time to get acclimated to the altitude.