Using the same code as shown here: https://jira.mongodb.org/browse/CSHARP-708 I noticed that after updating to the 1.8.1 driver via NuGet that all my unit tests were failing. I tracked this down to the collection.Insert<T>(document) was failing to give an ObjectId value even though the representation was set to ObjectId for an interface member... Before and after the call to Insert the Id property value was null.

If you need a sample, I can try to put one together.