The significance of Unity Windows Phone support is that it makes it makes it relatively simple to port Unity powered games to Windows Phone. In most cases it should be possible to bring a game to the platform in a matter of days (although polishing and release may take a little longer). For consumers this should mean that more high quality games will be released for Windows Phone. For developer it means than a previous untapped market is available for their existing and new Unity powered projects, offering the potential for greater revenue.
From the blog post:
Unity 4.2 comes with three new platforms: Windows Phone 8, Windows Store apps and BlackBerry 10. That’s right, we’ve doubled the number of mobile platforms Unity supports! Now it’s up to you guys to create new games and port existing titles to these platforms so even more people can benefit from your creative talents.
In Unity 4.2, all users of the free version of Unity can publish to any mobile platform they wish, be it Windows Phone 8, Windows Store, iOS, Android or BlackBerry 10 without it costing a dime. In addition, Unity Pro users can use the Windows Store Pro deployment option (which includes the Windows Phone 8 and Windows Store apps platforms) absolutely free of charge.