Hi All, long time no see...
It appears that this hasn't been discussed here yet, but I can't imagine getting a more informed interpretation anywhere else...
http://www.dxomark.com/index.php/Publications/DxOMark-Insights/F-stop-blues
In a nutshell it claims that lenses with more aperture than about f/2 are useless on digital because the light comes at such angles that the sensor doesn't record it; camera manufactures hide this fact by raising the effictive ISO to offset this loss of light.
Now I know that DxOMark is not uncontroversial regarding the evaluation of lens performance, and have got a reasonably good idea of why that is the case, but it does seem that they are on to something here — unless somebody puts me right ;-)
Colin