While rooted Android users have been taking screenshots on their phones for a while now, stock, non-rooted owners have been left out of the fun (there are some notable exceptions to this rule, like the EVO 4G). No longer, according to Paul O'Brien, one of the visionaries in the Android community, who posted the following in reply to Cyanogen (aka Android god):

We haven't been able to confirm what exactly changed in 2.3.3, but according to Android Central, screenshots are now possible without root "because of some changes in the way the SurfaceFlinger service handles what it captures from the framebuffer."