Many confused answers here... Eruditass got it right, it's all about the viewfinder. Actually it's mostly the "ground" glass, which is not a ground glass anymore: it's a microstructured glass, optimized for light transmission with slow lenses, not for ease of manual focusing. Something a bit like a Fresnel lens. The eyesight, has nothing to do with this problem, nor the viewfinder coverage, nor the pentamirror or whatever.

Ken Rockwell suggests a simple experiment: "Look through the front of your fast lens at the focus screen. It's black outside the area of the lens that corresponds to f/2.5!". Try it! You will clearly see that no light comes through the outer part of the lens. If light cannot travel one way, it cannot travel the other way: only the light rays that hit close to the center of the lens can get through the eyepiece.

If you want a focusing screen optimized for actually focusing... you may try one of the KatzEye focusing screens. Never tried myself.

Edit: As a followup to Matt Grum's post, here is a picture of a 85/1.4 seen from the front side:

On the left: the lens alone (with my girlfriend holding the aperture open). You can appreciate the extra large entrance pupil (~ 61 mm). On the right, the lens on the camera. Here the camera is holding the aperture wide open, but you only see light coming out from the center of the aperture. It's roughly f/2.8, although the borders of the effective aperture are not very well defined.