Michael,
While agree with the concept of tailoring project resources and level of effort to the size and associated risk of projects (as I advocate a similar methodology), I'm curious.  Using this train of thought that higher risk projects garner more attention and resources, it would stand to reason then that larger project would be more successful then small or medium size projects.  Judging by the number of large high profile project failures, it would seem that the tailored resources approach is not widely used or is also not the complete answer to why projects fail.  (I suspect both are the case)
 
Are you aware of any studies that have measured the success of projects that applied a tailored or graded approach?  My guess is they have not been conducted yet.  As an IT Project Manager and consultant in the area of Project Management processes and controls, I feel that the reason for project failure extends well beyond just applying an appropriate amount of resources or even obtaining clear scope and requirements.
  
I don't have the answer either, but I think this topic needs further study by groups like Standish, Gartner or even PMI.  This is a pernicious problem not only in the IT industry but for projects as a whole for business. 

It would be interesting to conduct a study and perform a comparison between projects that apply a tailored approach to project management and projects that have all resources and corporate standards applied to objectively measure which method has a higher success rate in meeting project goals and objectives.

Thanks for sharing your thoughts on this topic.
John Winchell