Will start testing it any day.. :)

I decided that it was easy enough to code this my self.  I've uploaded two diff files, one for lib/cyradm.php and one for newaccount.php.  Apply these two patches and add a line like:
$AUTOCREATE_MAILBOXES = array("INBOX.Junk", "INBOX.Sent", "INBOX.Trash", "INBOX.Drafts");
to your conf.php file. 

A few notes:
You must specify the mailbox names as they would appear to the client.  This means that if cyrus's altnamespace option is yes, an item in AUTOCREATE_MAILBOXES would be "Junk" rather than "INBOX.Junk".  And if unixhierarchysep is yes then it would be "INBOX/Junk".  This is necessary because the mailboxes are not just added but subscribed to as well, and subscribing can only be done by a client connected to his IMAP account (not a cyrus admin).  Thanks to Tim Schaab (tim at madtown.cc), for whoms post to the mailing list this is based.

Check out Hsys-Mailadmin, an alternative to webcyradm:
http://www.hurrikane.de/de/mx/mailadmin/overview.html

You can define that structure in the config-file. Don't be afraid of all the German text on that page - the tool itself is localized.

Would it be possible to add some functionality to Web-cyradm so that it sets up mailboxes and rules when the accounts are created?

I was thinking in the line of when a mailbox/account is created it should create a structure like:

.INBOX
.INBOX.Sent
.INBOX.Trash
.INBOX.Spam

And on top of that set up the rules so that sent mail will be moved to INBOX.Sent and deleted mail to INBOX.Trash etc.

I'm about to set up a mail/web hosting site, and that functionality would be really nice to have, instead of having to run cyradm all the time for all accounts...

Anders.