Screenshot of the Nexus One from adb?

A vastly easier solution for ICS is to use the following from the command line

adb shell /system/bin/screencap -p /sdcard/screenshot.png
adb pull /sdcard/screenshot.png screenshot.png

This'll save the screenshot.png file in the current directory.

Tested on a Samsung Galaxy SII & SII running 4.0.3.

Actually, there is another very simple ability to grab screenshot from your android device: write simple script 1.script like this:

# Imports the monkeyrunner modules used by this program
from com.android.monkeyrunner import MonkeyRunner, MonkeyDevice

# Connects to the current device, returning a MonkeyDevice object
device = MonkeyRunner.waitForConnection()

# Takes a screenshot
result = device.takeSnapshot()

# Writes the screenshot to a file
result.writeToFile('1.png','png')

and call monkeyrunner 1.script.