Yup, the way Autofill currently works isn't very well through-out. I prefer the way iCab Mobile implemented all this. The dedicated section from my Web browser roundup showing / explaining this (the link is an image URL - click it to see the screenshot):
I’ve been asked for more info on how form autofill differs in the built-in OS3 Safari and iCab Mobile, the most recommended third-party browser, mostly because Safari’s approach seems to be less awkward (fewer taps needed to save the contents of a form and to retrieve them). The difference is the following: while Safari doesn’t allow saving the contents of arbitrary forms, not even some with passwords, iCab does allow this. Give a try to for example
http://www.winmobiletech.com/form.html or
xda-developers - Powered by vBulletin. Try entering anything in the fields and, then, press Submit. No “Save password” will be displayed; that is, you simply can’t save the contents of these forms.
Do the same with iCab Mobile: save the form content and, after reloading it / returning to the page, make it fill out the contents. It’ll be restored in both cases:
http://winmobiletech.com/062009iPhoneWebBrowsers/IMG_0204.PNG.jpg
All in all, iCab Mobile is much better at restoring the contents of
any form, not just a few select ones.