This is the answer they gave me when I asked them about the release of this game:
We are happy to hear you are excited about Angry Birds Stella - so are we! Yes, I'm happy to let you know that we have plans of releasing Angry Birds Stella for Windows Phones, too. Unfortunately I cannot disclosure any information about the release date as for now. Thank you for your patience!
I got this answer when I asked for support of Rovio Accounts (save progress in cloud) on WP:
When it comes to the Rovio Account feature, unfortunately it is indeed not yet available for Windows Phones. However, it is our goal to get the Rovio Account feature to as many devices as possible. We are constantly looking into opportunities to bring the account feature to new platforms, but cannot give out release dates or future plans at this moment.
I think we won't see the release so soon as we had expected.
Maybe late October...