According to voices Intel will eventually launch a dual core Atom for netbooks, the Atom N500.
I bet it will be a variant of the Atom D510 (1.66GHz clock with Hyperthreading), probably power optimized it means that at least the Enhanced SpeedStep will be supported, allowing dynamic clock and voltage scaling for the CPU core(s). To be noted that Enhanced SpeedStep is neither supported in Atom 330 nor in D510, it is supported only in the mobile variants (e.g. Atom N270, N280, N450 and N470).
This is definetly a good news, it will help improving netbook user experience. On the other hand don't expect a huge performance boost: my assumption is that the additional core and HyperThreading will help a lot improving "just" multitasking performance, single thread performance will be limited by the 1.66GHz and the simple in-order architecture.
Why is Intel doing it? I think that this move is Intel's answer to the dual core ARM Cortex A8/A9 based netbook and tablet PCs...