Hi all,
Plenty of people have asked about YNAB developing a Windows Phone app, and our answer has always (unfortunately) been that it's not within our budget. YNAB (the business) is run using a budget kept in YNAB (the software). If you run a business too, you may be interested in how we use YNAB to plan how our expenses work. You can sign up for our (free) YNAB for Business email course here: http://www.youneedabudget.com/business
Even though a Windows Phone app hasn't been within our budget, one of our enterprising users has developed an unofficial YNAB app for Windows Phones. It's called "Divvy". We can't support Divvy (we have no access to the source code, so we can't make any changes to it), but Tim is a great guy, and you can get more information about it, including where to download it and how to get support here: http://www.youneedabudget.com/blog/2014/divvy-for-ynab-is-here-an-unofficial-windows-phone-app-for-ynab/
Cheers,
-Ian