So let's reimagine that six-person dinner bill. Right now, one person pays and the others give her cash, or promise to. In the future, one person might pay while everyone else at the table whips out a smartphone and transfers her part of the bill to her, right there and then, at least if they have her account number, e-mail address, or cell number, and if everyone has a Visa. There is still the question of divvying up the bill. But at least the payment part is easier.
Correction, March 25, 2011: This article originally failed to make clear that PayPal does not charge a fee for person-to-person transfers between bank accounts. It does charge a fee for person-to-person transfers made via debit or credit card. (Return to the corrected sentence.)