Today, Skype is delivering on a promise that it made back in July of last year. That's a long while to wait for an update, but this one is pretty major. In short, the team has carefully crafted an update for Skype for Android that allows it to run in the background with virtually no impact on battery life. That's what they're claiming, anyway.
Essentially, the application (and the service as a whole) had to undergo some major re-thinking, which led to a "transformation" of the way Skype actually works.