The programming of multithreading will enable the use of more than one core, so there is no need to write new code to use 2,4,6,8, whatever amount of cores. If you look at the article from anandtech you can see that the quad core gets twice the perforamance rating of a dual, and four times that of a single, as expected. This really impresses me because I thought it would end up like SLI and not necessarily give perfect ratios of performance.
As for other quad cores coming out, they will be very expensive due to the fact that the 65nm manufacturing process is difficult to fit the cores onto. Intel has been working with 45nm for some time but it is not ready for implementation into desktop CPUs. When that comes out they will be cheaper, but that is not til like some time 2008.