Virtual objects are one of the ways, publishers and developers are looking to employ in terms of enabling micro-transactions in their future iPhone and iPod games.

Indeed, this week web trading card game Urban Rivals revealed it would be selling multiple card packs on the App Store for $9.99 for its iPhone app.

Now Digital Chocolate has revealed its own system.

Called NanoStars, these are characters which will operate differently depending on which game you use them in.

CEO Trip Hawkins explains the concept thus on his blog:

"Suppose you bought a Batman virtual character in this kind of system. Hypothetically your ownership of this 'virtual genetic code' could unlock a limited edition ringtone or screensaver. And a hooded black cape with pointy ears that your avatar could wear in a chat room. And you could furnish a virtual home with your characters, where your Batman might turn into a stunning Batmobile that sits out in front of your house.

"Then you could take Batman into various games.

"Perhaps in a Batman adventure game he plays himself. He turns into gorgeous virtual trading card that allows you to make a cool move in an online trading card game. In a soccer game he could be a really quick goalie who plays better in night games, and has a black mask. In a role-playing game he could be a magician that can turn into a bat."

The first game to use the concept will be an online trading card game called NanoVerse Castles, in which the NanoStars will operate as modifier cards to strengthen your deck.

There will be more than 100 NanoStar characters at launch, with more added over time.

Some will be available for free, and you'll be able to buy and trade them via the virtual stock exchange Digital Chocolate will be setting up. There will also be a air miles-style reward program that will unlock new features the more you play with your characters.