http://www.ex.ac.uk/~RDavies/arian/emon
A nice summary of the issues with digital cash that arise because bits are so easy to copy (unlike physical currency). Covers topics like online vs offline systems, and identified vs anonymous systems. The latter comparison is the more interesting, because with fully
identified currency, the life of a "bill" is perfectly tracked. As the author points out:
"By the way, did you declare that $20 bill your Grandmother gave you for your birthday? You didn't? Well, you won't have to worry about forgetting those sorts of things when everybody is using fully identified e-money. As a matter of fact, you won't even have to worry about filing a tax return. The IRS will just send you a bill."
EFF has several articles on digital cash. I reccommend the double_spending.articles because it's a nice mix of overiew plus some details and some math so that it doesn't seem like a bunch of hand-waving.