Yes, there is a new native layer called "Windows Phone Runtime", which is a partial, semi-overlapping subset of the Windows Runtime found in Windows 8.

However, it's partial and incomplete, and mostly targeted at game developers - to access most of the features of Windows Phone 8, you need to write your apps in a managed language, accessing the .NET API for Windows Phone, which is pretty much the same as it's always been, with a few minor incompatibilities, even in so-called "compatibility mode" for WP7 apps.

EDIT: That was a hideous run-on sentence, but I kinda like it.

I had meant dev tools as in the API and SDK code written by Microsoft. There is no big change in the app developer facing dev tools, but internally the kernel has shifted, thus Microsoft developers needed to port the same API to a different kernel, so that's what I meant by a big change.

I've seen the current state of the SDK and API, and while I understand porting to a new kernel was probably a lot of work for them, it makes even less sense considering that most of the WinRT APIs available in Win8 are not there in WP8 - it leaves me wondering what, exactly, they hoped to gain; so far it doesn't seem like a win for developers, and you can't write a full-featured native app for WP8. Maybe you'll be able to for WP9 (if the platform survives that long).