Geoff
Here is my 60/40 bike. 60% pave, 40% dirt. It is a road bike, with 8cm of drop and I think 60.8mm of trail, I have run everything from 25mm to 33mm tires on it and it has room for about 38mm or so but I haven't needed to go bigger... Right now, I have Challenge Eroica tires on and I love them but, I have not been in the dirt yet.
So, it really becomes a queston of what you want to do. Dirt roads in Colorado are pretty easy on a road bike with +32mm tires. Doable on 28mm, but with as sandy as it can get I prefer a little wider tire. There isn't much gravel, not like the flint hills but if you want to do stuff like that a bike that can take 42mm would be nice.
I think the Black Mountain Bikes are very nice. Also, consider some old vitage steel like a Specialized Expedition or Raleigh International.
mIKE