After upgrading to Ruby 2.0.0, bundler 1.3.0 and rubygems 2.0.0 I get the following warning when I start my Rails 3 app:

**Notice: C extension not loaded. This is required for optimum MongoDB Ruby driver performance.
You can install the extension as follows:
gem install bson_ext
If you continue to receive this message after installing, make sure that the
bson_ext gem is in your load path and that the bson_ext and mongo gems are of the same version.