Android 2.3.3 Gingerbread allows screenshots with no root required
According to Android Central, that has changed in Android 2.3.3. Rooting will no longer be required "because of some changes in the way the SurfaceFlinger service handles what it captures from the framebuffer." The non-geeky version: Android 2.3.3 isn't as stingy with allowing apps to intercept what's being displayed on the screen.
Their post also jokes that Android users can now give the raspberry to their WP7-toting friends, but let's remember this change is part of Android 2.3.3. Many devices don't have Android 2.2 yet, and even less have 2.3.X -- so it's going to be a while before root-free screenshots become the norm in the Android world.