Why Silverlight+XNA? What inherent advantages does it have?
Anyway, no, your strategy won't work: Windows 7 requires x86 hardware and there is no low-power x86 hardware out there, you can't build an x86 tablet with reasonable 3D performance that has more than a few hours' battery life and with a diminutive form-factor. The iPad can play 10 hours of video non-stop, you can't do that with x86, and you know the next iPad will be even thinner.
Why SL + XNA? Because this is what is being used to create WP7 apps, the exact type of apps that people want on a tablet. Creating desktop apps to run on a tablet is just not what people want. In addition, if developers can easily target both WP7 as well as the tablet UI with the same code, it would mean that there will be a lot of apps on the tablet that work well on that form factor.
I see your point about the hardware issue but this is something that can be addressed in the future. BTW, are you saying there will be no tablets running Windows 7? Because they will run into the same issues you mention, right?