The Battle.net balance will let players convert cash into Blizzard bucks, which can be used to buy Blizzardy bits like World of Warcraft game time, realm transfers and name changes, or even digital downloads of StarCraft 2 and Diablo 2. A post on the Battle.Net site says that the Battle.Net Balance is now up and running for everyone with a Battle.Net account.
It's also a very important piece of infrastructure that'll prop up Diablo 3's in-game auction house. The system will let you put the profits you've made selling off useless loot towards other Blizzard game services. We can expect to see Battle.Net balance syncing up with the Blizzard Arcade in future. That could let players charge for mods they've made using StarCraft 2's editing tools. That's set to arrive alongside StarCraft 2: Heart of the Swarm.
The Battle.Net Balance FAQ mentions that any money put into the account can't be refunded, but adds that "Diablo III players will be able to make purchases on the currency-based auction house directly (without using their Battle.net Balance)." You won't be able to transfer money in your Battle.Net balance between accounts, either. If you log onto your Battle.Net account management screen, you'll see your current Battle.Net Balance displayed in the top right corner.
Battle.Net balance was tested using the Diablo 3 beta. It's full release lays the groundwork nicely for the game's full release, which may be sooner than we think, despite recent talk of major changes to some of D3's core systems . There's still no release date, though, so we'll have to go back to strengthening our clicking fingers in preparation for the monster slaying to come.