My guess is that it will take a while for the full support to show up for the multi-core processors (perhaps similar to 64 bit support lag we saw during the first two years of their existence).
I know the Adobe's new CS3 suite is taking advantage of the dual core processors where it is efficient to do so.
The quad core would appear (at least in the immediate future) to be applicable for servers and those companies who write their own code.
I did read an interview with one of the Intel developers that indicated that it is unlikely they will double again to eight cores on a die. I believe they are going to look at the manufacturing method employed by AMD, producing a multi-core processor on a single wafer. This apparently is more efficient from a production standpoint, as well as being more energy efficient in some way. The reluctance in going that way immediately is apparently due to heat dissipation issues.
We can still thank AMD for keeping the prices down and the development rolling. According to some experts - AMD's existence has prevented us from currently being stuck running Pentium III's.
I for one would like to see the industry flatten out a bit where Vista would be the last major OS provided by Microsoft and just receive service packs and allow the hardware to become more advanced without the big price hikes.
Perhaps just a dream, but worth the thought - IMO.