You can click on the Enable AutoLoad link to have Roundcube auto appear after 5 seconds on each login. This is designed to allow the mail user to have access to the other options in the mail interface which can greatly improve their mail experience.
For example, Mail filtering allows them to set up filters/rules on their account so they don't have to configure them in each email application they use. Mail users can also set up filters to send mail to another account through this interface rather than having to ask the site admin to do so.
There's not currently a way to edit the login page but you could put in place a custom page using /scripts/postupcp as putting a custom page in /usr/local/cpanel/base/webmail/x3/ will result in the custom page being overwritten after any update.
To 'hack' a custom page into place See my later post for a cross-browser JavaScript approach:
Create a file called /root/roundcuberedirect.html that contains this:
Code:
<cpanel Email="check_roundcube()">
<meta http-equiv="refresh" content="0;<cpanel print="$CPVAR{'roundcube'}">/index.php">
Then, create a text file called /scripts/postupcp that contains this:
Code:
#!/bin/sh
(cp /root/roundcuberedirect.html /usr/local/cpanel/base/webmail/x3/index.html)
After those are created, the html file containing the meta-refresh will be copied in place of the webmail home page each time you update cPanel. run /scripts/postupcp to switch the files and you'll be good to go.