Is there a database agnostic nosql framework for .NET?
No these things are too different and too specific (at least right now). If you wanted something really simple, like just a wrapper on an object that is only accessed by ID, then you may have a hope. In fact, if you look at NoRM, it may be possible to adapt that to various providers.
However, outside of a small core set of features, these "NoSQL" databases are quite different in many regards. I mean, how do you implement the various map/reduce functions agnostically? How do you implement atomic operations when they support different atomic operations?
Either way, we're way too early in the NoSQL life-cycle to have an agnostic framework for all of this. Azure basically dropped their NoSQL offering in favor of "hosted SQL server". MongoDB is maybe 20 months old, CouchDB is still on version 0.11.x, SimpleDB is less than 24 months old, Cassandra is on version 0.6.2 and has maybe been in regular use for a couple of years.
We're just not there yet.
There are LINQ providers for MongoDB but I don't think that there is a generic .net linq provider to 'all' nosql db's .
Some people have contemplated about a generic nosql query language: http://nosql.mypopescu.com/post/731261002/a-common-nosql-query-language