Wikipedia articles relating to post-Vista software contain images of windows using Aero that respect the original transparency layer instead of capturing the blurred render of what's underneath.
WindowClippings is capable of this but it's not free. I doubt a paid application would be a standard for Aero-ed window screenshots on Wikipedia, so what do they use?
Update: I apologise that it's impossible to notice the alpha layer on the above image. In the photo below I've changed SuperUser's background colour to prove it is there.