This is an older original post, but I'll go ahead and add a reply anyway, as I think the more info about vegetarian options out there, the better.
It is now quite easy to get vegetarian food along the Camino, but you do have to be specific about how YOU define vegetarian. Otherwise you might find a trout on your plate, complete with head and tail. In many cases the nutritional value of the meal may be lacking, but you certainly won't go away hungry. And if you are watching your cholesterol, watch out for all those egg-based veg solutions!
The following albergues are specifically vegetarian: Pieros (2 km after Cacabelos), Las Herrerias (1 km after Ruitelan), and La Faba (not the German-run one). Ruitelan will provide a veg option on request, as will all other albergues that serve a meal on-site. Many albergues have a kitchen, so the options are as broad or limited as what's on offer at the local shop.
Updated 2013: Two additional vegetarian albergues not to be missed: Albergue Verde in Hospital de Orbigo, and Albergue Ecologico El Beso in A Balsa, 1.5 km after Triacastela.
There is a fantastic vegetarian restaurant in Pamplona, called Restaurante Sarasate, on c/San Nicholas, only 50 meters off the Plaza Castillo. They now offer a pilgrim price on the regular Mon-Sat lunch menu, at 9.50 euros (not sure they are offering the pilgrim price as of Fall 2013, but do ask).
There is a vegetarian restaurant in Burgos, called Gaia. It is located just opposite the cathedral, near Hotel El Cid.
In nearly any Spanish city, you will find an Italian restaurant, which will have veg options. My favorite (proper) pizzeria on the Camino is la Competencia, with locations in Burgos, Leon, and Ponferrada. Burgos has a number of places that serve falafel sandwiches/salads, and a new place has opened in Melide. And of course Santiago has plenty of choices, with my favorite being Cedro's Restaurant for falafels and Thai food.
No fear, vegetarians! Plenty to choose from