Of the places I've been in my life that would have interesting and challenging dirt roads in the spirit of the great european bike race courses, I nominate rural Boulder County, CO. When I lived there in the 80's they had lots of dirt roads, many roads with long grades in the 4-9 percent range, and shorter grades in the 9-15 percent range. Add to that elevations in the 8,000 to 12,000 foot levels and you get a real challenge!!!! Boulder remains a road bike mecca so support is there. Gotta watch out for those afternoon thunderstorms though.
Michigan has a lot of dirt roads, too. Unfortunately they are too flat or too short.
I would guess that Smokey Mountains (N. Carolina, Kentucky, Tennessee) and Appalacians(sp?) in West Virginia would have some interesting routes, too. Do we have any members who can comment on these or maybe the Ozarks in Missouri?
The Marin Headlands on the other side of the Golden Gate from San Francisco offers lots of dirt roads (fire roads and single track) but they are steep. I was evaluating a 100k route there. The route we did was about 57 miles. It included about 22 miles on dirt and there was 9500 feet of climbing
Todd Teachout
Hercules, CA