With the introduction of multi-cores, parallelism is going to be more important. One of the easiest types of applications to scale is one with small and independent tasks, for example a standard web-application. Requests can be executed concurrent, so increasing the number of cores, is going to increase the throughput (not the latency). When latency also needs to be improved things get much more complicated because you need to introduce parallelism on a much lower level. Web pages are getting more and more complex, so building a single screen is going to take more and more time if no extra parallelism is introduced.
The cool thing is that with the introduction of Rich Internet Applications (RIA), the type of request is going to change. Instead of having one big ‘build me the complete page’ request, you will have a lot of smaller and independent requests ‘please get me the newest value for this page component’. This means that RIA applications could scale a lot easier. I have no numbers to back it up, so don’t shoot me if I’m wrong.