The Switchback Surge from Scosche Industries

"Scosche Industries has announced the availability the switchBACK surge g4, an US$79.99 backup battery and kickstand case for iPhone 4."

It appears to be a pretty nice case, slim design and a kickstand no less. You can also get the free reVIVE app to use in conjunction with the battery pack so you know exactly how much time is available. The case is available for AT&T or Verizon iPhone 4s, and if you don't like using the app, it has 4 blue LED lights to let you know how much life is left in the battery. Its available now from Scosche and the app is available from iTunes. I'm not sure as I haven't tested it yet, but it appears you can even use the app to see how much time is left without having the kickstand case.