Wanted to follow-up since I've made it home. Despite a handful of naysayers above, camping was a success. Here are a few examples:
First night in Mexico. Camped within feet of a mostly-deserted road.
Few nights later found this amazing spot. I was about 100m from the road; the occupants of the three or four vehicles who passed all waved.
Same spot, different angle.
This was a last-minute find as the sun was setting and I was in the mountains, no towns nearby. As ldeikis said, all I could find was rocky ground, and this was the best I could do. As it turned out, it wasn't bad.
I got near Mazatlan and found this $5/night place directly on the beach. My first paid camping.
Dry riverbeds turned out to be a good place. Only trouble here was that heading into town, the big trucks used their Jake brake across the bridge. That, and it took me a little while to find a suitable path to get under the bridge.
This was in Sayulita, just north of Puerto Vallarta. $5/night. I liked the atmosphere so much, I stayed three nights, the longest of the trip.
After the first night, I moved my tent in the shade of a Palapa.
I scouted this place before dinner, marked it on my GPS, went into town to have dinner and some internet, then returned after dark. Well off the road, and nobody around.
I happened upon this perfect spot near Tequila. Down a very(!) steep one-lane, I came across a gated picnic area. Once again, didn't see a soul.
On the shore of Laguna de Chapala, I stopped at this bar and grill and asked the owner if I could set up a tent. "You want to camp here?" "Uh... yeah." I ate dinner and she offered me free lunch if I was still there the next day...
This was on the slopes of Nevado de Colima, somewhere around 11,000 feet. Very cold night. Probably below freezing.
Next day at the base of Volcan de Colima, as it shoots ash and steam into the sky. My best campsite ever.
Again, $5 for the night.
Sans tent. I was setting up a tent not too far away, a family stopped and offered me a cot. As nice as it was, I hardly slept. The village had barking dogs, crowing roosters, mosquitoes, and early morning activities.
This one required a water crossing.
Although there was junk all over, I asked the nearby business owner if I could sleep under the roof. No problem. I stayed dry in the heavy rain that night. (And dry gear in the morning is a Good Thing.)
So... although it sometimes takes some creativity and patience, camping in Mexico can be rewarding, safe, and much less expensive than other sleeping options.