Indeed... Intel's 'Hyperthreading' is a stab at multiple processors in a single core, albeit merely sharing the different execution pipelines, rather than dedicated seperate hardware and registers - but it's bringing it to the desktop....
Sun have the new UltraSparc processors coming out shortly with 2, 4 or more processors on a single die - I don't think it will be mass market for a while though. It won't be long before AMD and Intel have consumer hardware out either.
I think IBM are heading the same way with a version of the PowerPC architecture, too...