Integration test, sessions, and acts_as_authenticated

Just starting out with integration tests here. I am having problems keeping a session between requests, using acts_as_authenticated. Here is a sample from the test:

login_as :store_user
post "/order/checkout"
assert_response :success

The checkout method is protected with a before_filter :login_required. The user is not being logged in; the filter returns false and the assert fails.

If I printline some debug statements in there to inspect the session, I see that the user only exists in the session between the login_as and post methods. The session ID is the same throughout the test, but the "user" key disappears from the session somewhere during the post request. Is there something I'm missing?