Update: 29. Jan. 2012
Analog SLR viewfinder compositing in Blender 2.6
Node Tree for Blenders compositor to simulate an old non-autofocus SLR viewfinder focusing screen
(Click to watch the short demo video of the first test, using a scene from 'Vivre sa vie' Jean-Luc Godard 1962 with Anna Karina)
My old Pentax MX SLR was the reference. The compositing in Blender 2.6 is not perfect (can never be perfect) but it is very close to the real viewfinder.
viewfinder_final.blend 3.8 MB incl. all Images (29. Jan. 2012)
Node setup info screenshot