This was an implementation decision made during Microsoft's Windows NT kernel design, and based on our experiences with both processor vendors' hardware, it wasn't considered a "feature" to either company. Of course, it affected Intel in a much different way than AMD. The problem Intel had in Vista was one of power consumption. For every migration, you had to write-combine the Nehalem architecture's L3 cache, which cost power.
This changes with Windows 7 and a feature called ideal core. If a task's needs are being addressed by one core, the operating system will let you stay there. This means two things to Intel: first, you don't use power on the migration, and second, idle cores are able to remain in a C6 state. Purportedly, this migration fix alone will yield an extra 10 to 15 minutes of battery life on Nehalem-based notebooks, though this won't become a major issue until the mobile dual-core Arrandale launches later this year. Perhaps more interesting, though, is that processors without C6 will not realize this gain (including AMD's CPUs)."
