In what can be currently considered a mere rumor, there seems to be some indications that PayPal may be coming to Steam. The payment service has gone long unused by Valve, possibly due to a lack of buyer and seller security, in addition to the system lacking strong age verification means. Fear not, would-be and hopeful electronic buyers, there may still be hope for you yet.

In an interview with Valve's Gabe Newell, a German news site was the first to catch wind of this information. The news was later revealed in a rough English translation on the Steam forums.
While in the US and other parts of the world credit cards are really popular, in some countries credit cards are rarely used by anyone. One and probably the most important example (for Steam) is Germany, where only a few people have a credit card.
Steam currently only supports credit cards, no other payments are accepted. This is a huge drawback for people without a credit card. Nevertheless last month more Steam games were bought in Germany than in the US - according to Gabe Newell.
Certainly, nothing has been confirmed or announced yet about just how concrete the probability of PayPal being used on Steam is. There are still a number of obstacles to overcome before something like this would see the light of day.

Oh, and since I was browsing around the site containing the interview, I figured I'd link to the display booth for Valve from the convention. They do love their crates.