Hi, I'm not sure what you are asking? Do you mean the Zimbra Mobile webmail page, or the Network Edition module Zimbra Mobile that allows OTASync ?
I'm thinking you mean the webpage as the iphone does not support exchange just yet, so if I'm right, you mean when you launch Safari on the iphone and point it at
http://mail.domain.tld/zimbra/m/ (iirc) you get the login box and you want a way to have the phone remember your username & password and go right past the login box.
Hmm. I'm going to go out on a limb and say it's not possible.
As far as I know, the Safari browser is closed when you navigate away from it (to launch something else on the phone) which would mean that a new session ID is created when you reload, which would log you out of the client.
If there was a way to pass the username & password to the server within the URL, then that could work but I think I remember trying something like this a long time ago and found it couldnt be done.
Others may have a suggestion, but I have a hunch that what you ask cannot be achieved.