When I was looking into visiting Vimy Ridge I had difficulty finding good instructions on getting there. It didn't occur to me to call Veteran's Affairs, which would be a good place to start. But anyhow, here's my insight, should anybody happen to google by.
Step 1: Get your ass over to Paris.
Step 2: Go to Gare du Nord, and catch a train to Arras. If you wish to reserve a ticket in advance, it can done at any train station.
Step 3 / option 1 (recommended): take a cab from the Arras train station to the Vimy Ridge national historic site. Approx cost: €20 one way.
Step 3 / option 2 (not recommended unless you have little money): take a bus to the Vert-Tilleul stop (the last of the three stops in Thelus). Walk approx 1 km up the highway to the turn off to Vimy Ridge. Warning: it is a busy highway and there are no shoulders. Follow the road another 3 km-ish to get to the park. Cost: €1.
The bus does not run very often, and it does not leave from the train station in Arras either. The cab is by far your best bet. Keep the phone number for the cab (06.17.59.7101). When you are ready to leave you can call one out to get you, or ask one of the nice folks who work at the park office to call one for you.
If you are really lucky, like I was, you might get a free lift back into town by a kind French gentleman named Monsieur Devloo who often gives Canadians rides to and from Vimy Ridge. [02/10/09 edit: Mr. Devloo recently passed away]
TO THE VALOUR OF THEIR COUNTRY-MEN IN THE GREAT WAR AND IN MEMORY OF THEIR SIXTY THOUSAND DEAD THIS MONUMENT IS RAISED BY THE PEOPLE OF CANADA
[03.2016 update: A commentor below gave the following numbers for taxis to/from Vimy: 06 69 34 74 75 or 06 58 23 36 37]