The mailbox may be a legacy mailbox.
You have a Exchange 2003 in the domain,rite?
you also created the user from AD users and computers ?
move the mailbox to exchange2007
If this was an upgrade from Exchange 2003 to 2007, check the properties of the mailbox owner’s account in AD. Check to see if “Inherit permissions” on the security tab is not checked. If it is not, then check it. We saw several of these from older accounts in which inheriting permissions was turned off on the AD account. Checking to inherit fixed the OWA issue.