Lots of details were announced about Windows 8 today at the BUILD conference.
One thing I heard quite a bit was "if you're a Windows Phone 7 developer you're now a Windows 8 developer too".
This is because you can use the same development tools & languages and share a design language heritage.
You may recognise this as being very similar to a phrase that was used a lot last year: "If you're a Silverlight developer your now a Windows Phone 7 developer." (Because both use the same develompent tools and languages.)
That statement was as wrong then as it is now!
They are not identical and so the statements aren't true.
It's like saying "if you know how to drive a motorbike you know how to drive a bus".
If you can drive a motorbike there are lots of things you'll have learnt that will be useful when it comes to learning how to drive a bus but that there are (I imagine and hope-I don't drive either) lots of important differences too.
If you've got some experience developing for Windows Phone 7 (or Silverlight) then it'll certainly help when you want to start building for Windows 8 but don't expect it to be exactly the same.
Also, don't expect any WP7 apps to run as is on Win8. At the very minimum you'll need to recompile but you should also look to make appropriate use of a UI (& UX) which is more appropriate to Win8. And yes, I totally expect this to mean you'll likely need multiple UIs to support the plethora of devices which Win8 will run on.
I'll probably write more on this in the future, when more Win8 details are available but don't expect this fundamental principle to change.