Yes, when we adopted him, the folks at the shelter thought he might be Dane and Mastiff mix. That pushed us to the top of the list because we have had Great Danes in the past and "experience" with the breed is something they were looking for.
After getting him, we started looking at different breeds and he matches up with some of the American Bulldog strains. He's got the personality of one, sweet, really smart and a little stubborn. Our vet seems to think the same.
Chances are he's a mix of something, but we decided that's what we would tell folks when they ask.