My colleague J.D. just posted that we've shipped to MSDN the Performance Testing guidance for Web Applications. This is the same guidance that was previously available on CodePlex. Looking at the TOC, this looks like the name is wrong, and it should be the "Hitchhiker's guide to Web Application performance" :) J.D. and team do an amazing job framing out all of the problems in a very clear and coherent fashion.
I wish I knew about this stuff when I was a web developer! (Well technically I still am one, just a little rusty around the edges)