During a press event today Rovio has lifted the lid on its latest venture, an in-app payment system with the perhaps inadvisable name of Bad Piggy Bank.

Bad Piggy Bank will allow carrier billing so that in-app purchases appear as charges on users' phone bills.

It's powered by Fortumo, an Estonian mobile payments service, and it'll be available on Finnish operator Elisa first, for Android phones. It'll get a worldwide roll-out next year, though no mention is made of other mobile platforms. Any developer will be able to use Bad Piggy Bank.

Rovio CEO Mikael Hed says, “Angry Birds has become the benchmark in casual mobile gaming, and a huge part of the whole game experience is convenience. With the help of Elisa and Bad Piggy Bank, we can introduce the same high level of convenience to purchasing our products and services.”

During the press event Rovio also revealed that Angry Birds Android will soon get an update, allowing users to pay for an advert-less game and the opportunity to buy the fabled Mighty Eagle. Unfortunately, this only seems to apply to people on the Elisa and Saunalahti networks in Finland for the time being.