It''s really simple... but you HAVE to render to your frame buffer, just because that is the way OGL works. But to quickly awnser your question, you use glReadPixels and get the data from your frame buffer and then save that to a bitmap. Their is an example of this in the book "OpenGL Game Programming" (a pretty good book), you get get some source (not quite sure if it will have the screen capture though) from glbook.gamedev.net chapter 7
http://romka.demonews.com/opengl/demos/win32_eng.htm look for OpenGL Bitmap...
it states: Demo, which demonstrates OpenGL rendering to bitmap, very usefull for all people, who is needed in offscreen rendering, for example when You want to do Your OpenGL output flow to AVI file or printer
I havn''t had a chance to try it yet, but it sounds handy