Firesheep has gone mobile. For those who may not remember, Firesheep is a Firefox extension, released in October of last year, which allows the user to access the social network accounts logged in on the same network. Developer Bartosz Ponurkiewicz adapted the Firesheep concept for Android.
The only prerequisite for FaceNiff is a rooted Android phone. Once installed, the app will tear through the Wi-Fi network for any accounts that are logged in. Protected networks aren't safe from this, as FaceNiff can access WEP, WPA, and WPA2 Wi-Fi networks.
FaceNiff cannot, however, access accounts that use https browsing, which encrypts information for a more secure browsing session. Facebook is not automatically on https. Users must enable it manually, and Twitter also requires users to manually activate it. Perhaps with the wider availability of an app like FaceNiff, https browsing will become standard.
Be sure to enable https browsing if you haven't already.