Form autocomplete on iPhone/iPad
Safari uses the form's fields name to detect if they can be autocompleted.  Unfortunately, some sites use names that are not recognized by the browser, which in that case does not fill them. 

This is not a safari issue, but more a website issue, and I think there is no workaround on the iPhone.

But there may be a workaround : I use 1password on both my mac and iPhone, and it is more "tolerant" than Safari for detecting fields.

The advantage is that, when I sync my iPhone app with the one on the mac, I can then use the iPhone app to autologin within Safari mobile. But it is a workaround, as I must save the form's content on the mac before being able to use it on the phone, and the detection is still not perfect.