Rails3.0.0Beta4 setting session store to database

I am porting a Rails 2.3.8 app to Rails 3.0.0Beta4. I can't get the
session to be written to the database instead of cookies.

I do the following in config/environments/development.rb

  config.session_store(:active_record_store)

The session store is set correctly - I verify that by dumping config
after the set.

What am I missing? Thanks.