I believe there is a 5 year statute of limitations on the DUI/entering Canada thing. I have a friend that happened to, and right before he was going to be allowed back in the country he got ANOTHER DUI.
He found out there was an appeal process he could go through... a bunch of paperwork and pleas and stuff that he could do, requesting to be allowed into Canada anyway. They had a hearing and he was denied, so who knows when HE will be allowed back in.
Also I will say that he found out that which border crossing he went to determined whether or not he was let in. Not every border crossing checked his record in the system, so occasionally they'd let him into Canada (he lives in WA state and could cross in a few different places). But if they ever DID check, they would turn him away. And like I said, he was GOING to be let in after 5 years... he just screwed up and did it again. It's a Canadian thing, not a US thing, so you have to run any appeals through Canada.