I will be bicycle/camping in France this July and I have contacted a couple of campsites and they say they only take reservations for a week or more. Is this the norm? I will not be in areas for more than a day or two at a time. Any ideas?

I don't know which campsites you've contacted, but since my house in France is not far from several campsites which I frequent from time to time to rent bicycles I do know that it is not normal for reservations to be taken for only a week or more except maybe in high season - July and August. A lot of these places have regular customers who come for several weeks in summer in high season, so maybe that's the reason you're getting this message. It's certainly not "the norm" for all of the year.
When are you traveling, and where, and which campsites have you contacted?

Go to Borders or Barnes and Noble bookstore (or Amazon.com) and get a "Camping and Caravaning in France" book. It lists hundreds of campsites with phone numbers, addresses and often web sites or e-mail addresses. It also mentions little oddities such as only accepting reservations for a week or more. If you have a tent, they will usually put you up for a day or so even if they are full. There's always a plot of grass someplace!

Camping in France is great. You should have a good time. Bonne chance!

Thanks to you for your replies. I was assuming that, if I show up on a bike, they would find me a little piece of ground somewhere to set up my backpacking tent. I don't need much.
I am planning to camp near Chartres, Chateauroux, Gueret, Limoges, Aurillac, Agde, Gap, le Bourg D'Oisans, and Castellane. I am roughly following parts of the Tour de France route and attempting to see the race at various points.
I am planning on staying in hostels in Tours, Millau, Nimes, Aix en-Provence, and Nice.

Maybe with the exception of Agde and Bourg d'Oisans, you shouldn't have any difficulty finding a spot in a French camping site. That is, unless you follow the Tour de France as it actually unfolds, in which case I would worry a bit...

Bourg d'Oisans has a wonderful camp site. It's outside of town and by a rushing river. We were there during the Tour de France one year and there were actually a few places open.

In addition to "Camping and Caravaning in France" you might want to get the "Lonely Planet" guide to France. It lists camp sites with directions to the campgrounds. It also has youth hostels and budget restaurants. We made our first trip with Lonely Planet and had a great time.

Sounds like you'll have a great time.