From the title I was really hoping this was going to be a story about the differences between cash and checks/credit cards/ACH.

In case you were equally disappointed I do have a pointer to an interesting article from PET 2004 about the anonymity of banknotes but sadly I only have my hard copy. The slides are here:

http://petworkshop.org/2004/talks/kuegler.pdf

If you have a springer account or (great google-fu and patience) you can get the full article:

http://link.springer.com/chapter/10.1007/11423409_8