C# 5.0 is all about making asynchronous programming a first class citizen of the language. Also, writing scalable server applications requires using only asynchronous I/O. This can only be done if the MongoDB driver exposes asynchronous operations for querying the database.

In order to make it compatible with what C# 5.0 will be using, those APIs will need to use the task based pattern, and return Task<T>.
For exemple,

public MongoCursor<TDefaultDocument> Find(IMongoQuery query)

would have an asynchronous equivalent:

public Task<MongoCursor<TDefaultDocument>> FindAsync(IMongoQuery query)

The resulting task represents a token for asynchronous completion of the operation.

The driver exposes lots of I/O operations, but having the most used ones (Find, FindOne, Insert...) implemented first that way would go a long way.