I read an interesting article in today's Dayton Daily News about a township which allows private open space conservation easements. http://www.daytondailynews.com/n/content/oh/story/news/local/2007/07/29/ddn072907easement.html
Essentially, the concept is that an individual homeowner commits, via easement, to preserve a certain portion of their property as open space (pastureland, farmland, etc.). This is an innovation insofar as most open space preservation schemes reserve the open space in a public or semi-public entity, like a townshuip or homeowners association. Here, the preservation is private, and, as the article states, is essentially a very large backyard.
The article seems a tad outraged at the concept of private preservation of open space. However, in its current context as farmland it is open space belonging to a private owner. Therefore, to the extent that open space can be "sold" as part of private development, I believe it could be an effective tool in balancing development and open space. However, to what extent people will pay to be obliged to maintain part of their property as open space remains to be seen.