Google apparently plans to take on PayPal, which is owned by eBay. PayPal is the only online payment system that has been successful, despite dozens of schemes, some of which were very well-funded. PayPal has been successful in part because it is relatively simple--it bypasses credit cards completely and debits or credits your checking account. It is fast and simple, and the online reporting of transactions is excellent. It's about the same cost as credit cards; the company charges 2.9% of each transaction (the person or company receiving the funds pays).
Despite Google's grandiose slogan, "Do nothing evil," the company has hewn a pretty straight path to try to capture every possible kind of online interaction, to the extent that it makes Microsoft look like a minor league player. One difference between the two companies is that Google seems to be trying harder to deliver quality with version 1 of new services, whereas Microsoft's strategy was to throw something fairly buggy out there and get users to pay for quality control by releasing frequent "upgrades."
Google's relentless quest to remember everything you have ever done online (my main objection to the company) seems likely to spill over into this new payment system, where Google will maintain a dossier of every purchase you have ever made--a nongovernmental entity with more information about you than the government. They'll link purchase information to the emails you have sent via their free GMail service. They'll tie purchases to Google searches you have made. Ads will show up based on maps you have looked at on their Map service.
Google is making a fortune customizing ads, but they are amassing too much information about us.