Which is why MS has had a lot of free developer events going around the place for both windows phone 8 and windows 8. I think you can also use the free edition of visual studio (express) to publish to both respective stores these days.

One thing that is confusing to me is why they didn't fully unify the phone and tablet (Win 8) stores; right now there's two completely separate stores, each with their own infrastructure of registration and app approval.