Well, I can't resist a good mystery/investigation/challenge.
Basically, the site is doing browser-detection on the server side. Depending on what browser string you send, you'll get something different. ToX, I would be willing to bet you're sending an IE browser string.
IE = it gives you an IFRAME.
NS = it gives you a LAYER.
It thinks K-Meleon is NS, but K-Meleon doesn't do LAYERs.
David, you couldn't find it because it's just one LAYER tag with a SRC attribute and the LAYER tag isn't anywhere near the box's location in source. It uses absolute positioning so they just wrote it in under the BODY tag. The page it loads to put in the layer (or iframe) is:
http://www.omnitelvodafone.it/_login_hp/welcome_hp.asp
Po, do you by any chance have javascript disabled? That page uses javascript to insert a different input field into the page based on your browser. No javascript, no input fields.