I think they do already have a bias towards faster websites, but as with everything else with google it's only speculation as things constantly change with these guys.
I'm not sure Google needs to cook that book. People like faster websites, so they'll get more link love, so they'll probably rise up of their own accord.
If I'm searching for something to link to - often a news story, if I'm writing for the datablog we run - then I'm going to open five stories in tabs and probably pick whichever one finishes loading fastest.
Yes, I agree - I really value a fast-loading website, and am very annoyed if the site I clicked on takes forever to load.
Your theory that it could happen naturally is interesting, but I don't see it working in practise - ie, I don't notice any correllation between pagerank and site response time. I am sure Google et al have the resources to measure it in a reasonably non-game-able fashion; I'd certainly welcome such a development.
