I usually leave the Mail application up and running all the time and sometimes on the left panel (where all the mailboxes are shown), the inbox of a particular IMAP server is grey'ed out. This morning the same thing happened. MobileMe was grey'ed out and one other IMAP Server. I didn't really pay a lot of attention to it until I tried to delete a mail message and saw that not only was the mailbox grey'ed, but the delete button was grey'ed too. The only buttons that were available was the "New" message button and the "Get Mail" button.
I tried to see if the IMAP server was offline, but everything was marked as online. I tried to "Rebuild" and the rebuild was successful, but still grey'ed out. I wasn't sure what to do at this point. So I ended up quitting the Mail application and restarting it; which fixed the problem.
I've seen this behavior several times and I still don't know what the problem is or how to fix it without quitting the mail application and restarting it.