When the time has come, we will release Windows Phone SDK 8.0
Microsoft built a landing page that will be redirected to the Windows Phone SDK 8.0 when it release.
Windows Phone SDK 8.0 is an app builder in Windows Phone 8.
I don't sure that an app that write today can work in Windows Phone 8, but Microsoft tell that you can feel confident that the code you write today will work on Windows Phone 8 devices, so developers can use an old version of it.
You can bookmark the Windows Phone SDK 8.0 landing page, so when the time has come, you will be easily to download it. We think in Nokia and Microsoft press conference september 5th or in Nokia World 2012. we will know much more than today.