I'm looking for a common data access framework that will provide portability across various nosql databases like SimpleDB, Azure Tables, Cassandra, CouchDB, MongoDb, etc. I'm building an app and would like my customers to be able to use which ever nosql store they want. 

In a more relational scenario, I'd use Linq over nHibernate or Entity Framework, but I haven't found an equivalent framework for nosql databases. All I've found is database specific API's even though there seem to be significant commonality. Does one exist? Preferably one with LINQ.