PROBLEM SOLVED for me!
Verified all credentials (for the 10th time) and there was no issue with server name, domain\username, password, SSL enabled, exchange server activesync enabled, etc.
Checked with our hosting service (Intermedia) and there was no problem.
Checked with AT&T (and no matter what those yahoos tell you or anyone else, you can use your personal $30 data plan), no problem.
Confirmed the inbox was 'clean' in regards to corrupt or strange messages, so that wasn't the problem.
It apparently was related to some of the comments above on number of messages over time... I have on average 2,500 emails in inbox, and another 2,000-3,000 across contacts, sent items, etc. for over 5,000 items (and this is after a recent cleanup).
Go into Settings>Mailbox, Contacts, Calendar and select the exchange account.
Confirm all appropriate items are ON (mail, contacts, calendar) and then switch 'Mail Days to Sync' to NO LIMIT.
Voila, it is working like a charm. But beware, the performance has taken a hit trying to sync this monster exchange account. Hope this helps!