When we had a brief play with the mahoosive Xperia Z Ultra, Sony showed off its impressive screen-writing tech using a ballpoint pen. It's meant to be able to detect anything from pens and pencils to keys and nails forgoing a real stylus, and it seems that's not just marketing bluster. You really can use anything including coins and a Stanley knife. Ouch.
There does seem to be a capacitive threshold though, as a plastic ruler doesn't work, and neither does a mechanical pencil. Probably something to do with minimum conductance. Sony reckons pencils over 1mm should work though, so that really is quite a fine point. Of course, when your screen is scratched to hell, you're not exactly going to be laughing. [YouTube via XperiaBlog]