The reason people have such trouble scaling web apps is that they think the above is true.

Could you elaborate? Are you saying that apps are CPU bound or that web apps have not been dealing with concurrency even before multi-core? Perhaps you mean that web app authors care about CPU usage too little, which may be true, but I still don't think that multi-core marks a huge change for them.

The thing to remember is that there are two sides to a web-app. The client and the server. The web-app I'm currently working on uses very little CPU client side, but on the server side the number of simultaneous users I can handle scales more or less linearly with the number of cores the server has (assuming enough memory). So personally multiple cores (and using the correctly) makes huge difference to me and the web-app I'm working on.

I apologise for not being clear: certainly multi-core makes a difference to the performance of web apps. My point is that most web apps don't need a drastic change in the way they are written to take advantage of this: they are already multithread or multiprocess to handle concurrent users. Contrast this to single-user desktop apps which have simpler threading models and are less likely to be "naturally" parallelised to take advantage of multi-core.