i've seen this problem on both WebOS and Windows Mobile.
it works fine over IMAP but doesn't work worth a lick over EAS.
i'd imagine it's something in the server, and how its passing the OWA XML to the device.
Anyone know of a fix? it's rather annoying and has been this way for as long as i can remember us using it (Zimbra 4?)... and this has been across a number of devices.