Yesterday, I recalled that I had bookmarked a recipe site some time ago that seemed to have a pretty broad collection of recipes for pizzas, as well as many other food groups. The site, called "recipe hound", is at http://www.recipehound.com/index2.html
(click on pizza in the menu on the left). I randomly checked out several of the recipes in the pizza collection and noticed that several were attributed to James McNair, a well-known cookbook author, and several that were attributed to a Pasadena newspaper. I also saw one that was attributed to Bon Appetit.
I found a link to the person responsible for the site, Stephen Mason, and sent him an email asking him whether he had ever encountered any copyright problems because of recipes of others being incorporated into his pizza recipe database. I mentioned to him that our site was considering its own recipe collection, which would differ from his collection in at least one material aspect--in that members and guests of our forum could enter recipes whereas in his case he has sole control of what goes into the collection (he provides an email link for people who want to submit new recipes for possible inclusion in the pizza database). I received a reply to my email this morning in which he said that he always tries to note the sources of the recipes in his recipe database but that he would remove a recipe from the website if someone objected. He added that this has never happened during the 8 years since he started recipehound. His advice for us is to "go for it."
If the forum does decide to "go for it", then I think the groundrules should be laid out quite explicitly. I don't think that having a recipe bank should be an invitation, for example, for posters to, say, copy all of Peter Reinhart's recipes into the recipe bank. If the recipes are already on the Internet, then they may be fair game. But, even in any such instance, if the author is known, then there should be attribution of the recipe to that author.
I personally like Foccacciaman's recommendation that the recipes be primarily recipes of our members, whether they are completely original recipes or modifications of recipes of others, and expressed in the members' own words (that is, don't copy someone else's expression). I think those are low risk situations. After decades of people playing around with dough and pizza sauce recipes, I would venture to say that there are very few recipes out there for pizza doughs and sauces that are not already in the public domain. After all, how many really new ways are there to combine flour, yeast, water, salt, a sweetener and a fat? (In any event, copyright protection wouldn't attach to a simple list of ingredients, only to an original expression of the recipe, like describing how to practice the recipe.)
I think what remains to be considered is the process that would be put in place to control what recipes go onto the site and how to communicate the groundrules to prospective posters.