From Android Wiki
A non-rooted Android device has no built-in capability for taking screen captures. If you want to grab snapshots of an app, you have two options, both of which require installation of the Android SDK:
- Run the app under the emulator that comes with the SDK. Then you can use the normal screenshot facilities of your PC OS to capture images of the emulator window.
- Enable USB debugging on the device, and use the DDMS utility from the SDK.
On some Linux systems, the DDMS window fails to show any devices. Instead, it outputs a continual sequence of “E/DeviceMonitor: Connection attempts: n” messages with ever-increasing n.
This is caused by a system setting which prevents IPv4 and IPv6 connections from being opened on the same network socket. To work around the problem, execute the following command in a root terminal window:
sysctl -w net.ipv6.bindv6only=0
Presumably this is a bug in DDMS.