economists estimate that only 8 percent of the world's currency exists as physical cash. The rest exists only on a computer hard drive, in electronic bank accounts around the world.

http://www.howstuffworks.com/currency6.htm

I'm work in a small business and about 90% of sales goes through PDA machines ...  myself.....well I dont have more than 10 quids in my pocket for ages ... every time I lost my debit/credit card .. I rush to the bank agency because I simple don't have enought banknotes with me...