I am now a Canadian citizen but before that, I was only a national of a Visa Waiver country, with Canadian PR status.
You just bring your passport (and PR card) and make sure to buffer at least an hour if it's a busy crossing for parking your vehicle and filling out the I-94. The PR card isn't actually needed to enter the US, but it's a must to re-enter Canada.
In fact, if the dollar is at parity, I suggest avoiding land crossings during daylight hours until you get your citizenship, because the border line up takes forever and then the I-94 wait on top of that can really ruin your day. I obtained my citizenship while the dollar was worth more than 95 cents US, and I estimate I immediately saved at least 90 minutes per border crossing thanks to my Canadian passport. When I got my Nexus card a year later, I saved at least an extra half hour.
It really sucks that Canadian PRs are treated differently from citizens at the land crossings, even if they are VWP nationals. Canada treats green card holders the same way they treat US passport holders, i.e. you just flash your documentation at the booth and be on your merry way, no need to get out of the car.
With the exchange rate at 81 cents now (as of Jan 2015), I don't think wait times will be what they used to be, so you may not really need 1 hour. But better safe than sorry :)