1Password kind of supports the iPhone:
You can sync a special bookmark to your phone that once pressed will show a translucent overlay window over the current webpage. In that window you tap the login you want (it displays any related to the current website's domain) and then enter your 1Password code (which can be as long or short as you want).
It will then autofill the login boxes on the webpage and log you in.
It works on most pages I've tried, you just need to visit them and store the password using 1Password on your mac, then sync the special bookmark.
It's not a full password manager replacement, but it gets the job done.
I guess a real "password autofill" feature would just be too risky on the iPhone since if you lose it anyone could access all your online logins.