i went a couple weeks ago. (Niagara Falls, NY, Niagara Falls, ON)
Both ways they asked for a passport. My friend and I both had it so I'm not sure what would of happened if we did not. The exchange rate kinda sucks right now though. On the bright side, we did get a hotel room @ the howard johnson which is in walking distance of the casino for $28 USD. That included $10 of chips at the casino and complimentary breakfast at the Dennys next door (breakfast was horrible).
The time I went before that (March), I think we both used our driver's license without any issues.