I had the same issue but only after doing a clean reinstall of RC with the updated Beta version. I did not have the issue with the first release of the Beta or the version I was using for a few weeks prior to the beta releaes (roundcubemail-0.1-20051021)
I changed the value below from TRUE to FALSE in main.inc.php and the inbox updates immediately and new items now appear after login.
// enable caching of messages and mailbox data in the local database. // this is recommended if the IMAP server does not run on the same machine $rcmail_config['enable_caching'] = TRUE;