Some days ago, while trying to answer a call I accidentally took a screenshot with my Galaxy Ace instead of answering the call.
I do not know if this works for all Android phones, but it works with my Samsung Galaxy Ace
I'm running Android 2.3.4 Gingerbread.
To take the screenshot just hold on the back button, and then press and release the home button while still holding down the back button.
Your images should go to ScreenCapture folder in your phone gallery.
Here is a picture of the specs of my phone.