This method is different from MongoDatabase.RenameCollection. One is an admin operation, the other isn't. I wrote an extension method that holds me over:

public static BsonValue RenameCollection<T>(this MongoCollection<T> collection, string newName)
{
var javascript = @"db.

.renameCollection('

');".Inject(collection.Name, newName);
return collection.Database.Eval(BsonJavaScript.Create(javascript));
}