When logging in to RoundCube, running the stable version of 0.2, the browser is redirected back to the login form with no errors.
The login appears to be successful and all tests in the installer were successful.
The 'temp' and 'logs' directories have been recursively chmodded to 777.
Have you checked both your web server and mail logs for errors?
did you see this? http://www.roundcubeforum.net/pending-issues/3739-roundcube-doesnt-log.html? session.auto_start Off should be in the .htacess file already but may be its not being applied for some reason.