I have only used a couple of larger land border crossings (en route from YVR to SEA, and Niagara Falls) and both these had a supply of the I-94W green forms waiting for you to complete. VWP is supposed to be available for entry at all manned border crossings from Canada and Mexico. If you want to save some time, fill it in ahead of time (e.g. by grabbing one at LHR check-in) Same process applies as at an airport, though you may not be fingerprinted in the same way as at an airport entry point.
At land borders you have to pay a fee ($6 in cash, US dollars only). This is included in the airport taxes when you fly to the US.
If leaving the US by road, make sure you hand the green departure card to the Canadian immigration officer at the border, who collect these on BCP's behalf.
Here is some more official information from, of all places, the US Consulate in Vladivostok:
Originally Posted by US Consulate website
Can I Enter at a Land Border from Canada or Mexico on the VWP?
Yes. If you are a citizen from a Visa Waiver country, you can enter the United States on the Visa Waiver program from Canada and Mexico. You do not have to show round-trip tickets. You do not have to arrive at the land border on a carrier (form of transportation) which has an agreement with the U. S. Government.
http://vladivostok.usconsulate.gov/wwwhvwp.html