This has happened to me twice now this week. Both times the Inbox icon for one of my accounts is greyed out when I get to that screen (the one that shows Inbox, Deleted Messages, Trash, etc.). I can't select the Inbox, can't view my mail. The same account both times, an IMAP account that works fine on my Mac. A reboot solved this the first time, but now I think it's a bug.
iPhone 8GB, v1.1.3
MacPro 2.66, G4 Dual 1g, 15" Powerbook 2.66, 8GB iPhone, iPod video,
Mac OS X (10.4.11)
This also happens to me. I have an IMAP account and it seems to work fine until I give it an IMAP Path Prefix. It then displays my folders but greys out the inbox and won't let me into it. If I receive a new message, it takes me to the Inbox, but as soon as I navigate up a level (to the folder view) it greys the Inbox out again and won't let me in. Is there any fix for this? I've tried deleting the account and re-adding it many times. My gmail account does not exhibit this behavior.
I'm having exactly the same problem and Google is giving me no answers. I doubt this has only happened three times since the iPhone launched so hopefully somebody knowledgeable here will be along to solve the problem.