Basically, I think there are two possible routes for Microsoft choose from when it comes to the netbook edition of Windows 7. Either they remove functionality and make it cheaper for OEMs (and thus, customers), or they add functionality and specific tweaks to make the netbook edition a better fit for the usage patterns of netbook buyers.
The latter route is the interesting one, because you can theorise wildly about what type of tweaks and additions Microsoft could make to improve the user experience of Windows on netbooks. If you look at Microsoft's competition in the netbook space, most Linux distributions tailor-made for netbooks include a special interface for launching the most common activities for netbooks, such as emailing and browsing; it's not far fetched at all to assume Microsoft is working on something similar. When it comes to tweaks, Microsoft may disable resource/disk-intensive services, to save battery power and disk space.
As interesting as the added-functionality-route may sound, I think it's more likely that the netbook edition of Windows 7 will have reduced functionality, a sort of Windows 7 starter edition. This would allow Microsoft to charge a lower OEM price for Windows 7, which in turn makes Windows netbooks more competitive against Linux-powered netbooks.
What kid of netbook-specific features would you want to see on a netbook version of Windows 7? What functionality could Microsoft remove from such a version?