A Rare App for Now
For the time being, the only credit cards that work with Google Wallet are Citi MasterCards, and there are some restrictions even there. You can use this tool to find out if your Citi card is eligible. If you don't have a Citi MasterCard or it isn't the right kind, you can use what Google calls a Google Prepaid Card. Essentially, you fund the Google Wallet with an existing credit card.
Then, you'll need a phone that runs Google Wallet. Here is the problem: there is one phone that does this, the Nexus S 4G from Sprint. That's it. Google, of course, wants to roll this out to more new devices, but we don't know to which ones and when, so, for the time being, you have to have the Nexus S if you want to be able to do wireless payment via Google Wallet and PayPass.
If you have a Citi MasterCard but not a Nexus S you can actually request a tag that sticks to the back of your phone. It doesn't use Google Wallet, but it does allow you to use PayPass terminals as though you had a PayPass card attached to your phone, which gives you some of the functionality of Google Wallet.
However, you will be missing some of Google Wallet's features (more on that below). The tag is also, at least going from what they have pictured online, an ugly-as-sin branding nightmare, so unless you want to show your Citi MasterCard pride like it's your school colors, the tag is best kept hidden under a case. More seriously, the tag would doubtless attract the attention of thieves. I'm hoping what is shown online is some kind of artist's rendering and the actual product offers the user more discretion.