I am currently moving my IT infrastructure to a new server. That included also an switch to Cyrus IMAP 2.3.8 - which comes with Fedora Core 6. After an smooth and short move I have seen - so far - on problem in combination with Thunderbird 2.0. I could move mails to the trash, but all "Delete Mail" buttons are disabled. :-(
It turned out that Cyrus 2.3.8 has a few more ACLs and to resolve that issue I had to add those ACLs to my mailboxes. You have a lot of users with many mailboxes? Jo Rhett has posted a short Perl-Script on the info-cyrus maillinglist http://lists.andrew.cmu.edu/pipermail/info-cyrus/2007-April/025878.html.