After months of delays, Google said that it is finally releasing support for in-app payments to developers at a press conference this morning at its Mountain View headquarters. This will help Android developers tap into the rapidly growing free-to-play segment of games and sell virtual goods.

Developers can bill customers in two ways — either through Google Checkout or through carrier billing to T-Mobile and AT&T. They can’t use other payment methods like Paypal if they want their work to be listed in the Android Marketplace. But if they want to sell apps in other stores, alternative payment methods are fine there.

The in-app payments flow is fairly simple for consumers. Tapulous founder Bart Decrem came on-stage this morning to demo it as he announced that the company is bringing its smash hit iOS title Tap Tap Revenge to Android. In Tap Tap, players can open the store and choose to buy a song to play the game with. Once they choose a song, a Google Checkout screen will pop up. If the user already has a Google Checkout account, they can choose a payment method and buy the song with a single click.

If they don’t, they have to register and enter basic information like their credit card number. Decrem said it took five days for the company’s developers to test and implement the feature. Google director of engineering said Chris Yerga said developers will get access to the SDK now, and then will be able to roll in app purchases out to consumers later this quarter.

Groups manager Eric Chu, who leads Android’s relationship with mobile developers, told us last week at InsideSocialApps that the platform would support in-app purchases by the end of the quarter.