I'm also having the same problem. I installed Roundcube 0.3.1 and my Session never Expired. So, after i installed Roundcube 0.2 stable surprisingly this session is also not Expired .
I'm not changed the Config on both 0.3.1 and 0.2:
$rcmail_config['session_lifetime'] = 10;
$rcmail_config['min_keep_alive'] = 60;
$rcmail_config['keep_alive'] = 60;
I've tried the [PATCH] Add option to avoid un-necessary updates to the session table.
But no use at all,
Finally, I saw the session table in the Database which keep's updating 'Changed' field (every 1 min). I think that's why the session never expired.