Session id unavailable if assets pipeline enabled in Rails 3.2.1

I have a Rails 2.3.2 application I am upgrading to Rails 3.2.1. I am
using ActiveRecord::SessionStore connected to an Oracle database using
the oracle_enhanced_adapter.

On my first attempt (creating a new, default Rails 3 application
copying in the application code, and fiddling with routes etc) I found
that most things could be made to work, but that:

request.session_options[:id] returns nil.

After some trial and error I found that disabling the assets pipeline
(enabled by default in Rails 3.2.1) results in a usable session id.
And if you think about how unrelated the assets pipeline seems to be
to session management, you will get an idea of just how much trial and
error was involved.

Is this something I should have expected in Rails 3? I could find
nothing online about it. There is nothing obvious in the code.

I would like to be able to use the assets pipeline at some point, as
it looks like something that is useful, so if anyone can shed some
light on what is happening I would be grateful.

I feel like I am missing something obvious.