I've always wondered just how accurate signatures are. I've noticed that my own signature varies widely depending on various factors.
Signatures written on paper are not all that helpful for a few reasons. First off, they are easy to forge. Second off, a single person might sign his name twice and produce two signatures which look very different to both the naked eye and some forms of analysis - hence not accurate. Where they actually are accurate, however, is when written on pressure sensative pads (such as those seen on new-fandangled credit card swipers). If you were to do an analysis of the pressure and speed at which the signer signed various parts of the signature, you would actually produce some very reliable information. This is because even when you sign your name in slightly different manners you have the tendancy to use the same speed/pressure on certain parts of certain letters. Personally I would just use digital signatures...but calculating hash functions on the back of your resteraunt receipt is never fun. Its also difficult to fit a 256-bit output on that miniscule "sign here" line.