If anything it seems like Windows 8 will be a better fit for tablets than WP7. I'm not sure if porting Windows 8 "down" to tablets or porting WP7 "up" to them really makes a difference.
There is currently no reason why they can't have apps written for WP7 running on Windows 8 tablets, or at least make it easy to port existing XNA and Silverlight apps over. WP7 might have a nicer UI, but it seems like MS is giving Windows 8 full touchscreen support.
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If anything it seems like Windows 8 will be a better fit for tablets than WP7. I'm not sure if porting Windows 8 "down" to tablets or porting WP7 "up" to them really makes a difference.
There is currently no reason why they can't have apps written for WP7 running on Windows 8 tablets, or at least make it easy to port existing XNA and Silverlight apps over. WP7 might have a nicer UI, but it seems like MS is giving Windows 8 full touchscreen support.