Yeah, alert boxes aren't great I admit. I was aiming for the bare minimum of functionality but I could perhaps add nicer popups using script. It detects changes in browser window size and tries to redraw the button pad accordingly; and so should work well on mobile screens too though there are some width issues on iPod Touch.
(Apart from the fact that it allows decimal numbers like 127.0.0.1 and uses annyoing alert() boxes for error handling.)