I've been battling with this for a client for a couple of weeks now. The configuration is slightly unusual as the ES is sitting behind a NATed firewall but I'm using the virtual server and open ports features on the router to pass through the necessary traffic.
As a test, I've attempted to see whether OWA will work on the iPhone, which it does - but only for a test user accouunt. If I try to access OWA with the actual user's credentials I keep getting a HTTP 404 (file not found) error.
If I configure the iPhone to synch using Exchange, it just replies back with "server connection failed", whether I try the test user's credentials or the actual users. It does display the certficate message which I click Accept, then the 404 error appears.
any pointers on this would be much appreciated!