Waze stated previously that it had no plans to develop for Windows Phone 7. You can assume it's currently the same situation for Windows Phone 8.
Everything I have seen regarding SmartPhone market share and sales shows that Windows Phone has not been making any significant gains. It's currently a two horse race Android and iOS. All other platforms are currently insignificant.
Gartner's data published in November 2012 shows Microsoft Windows Phone at 2.4% of 3rd Quarter 2012 Worldwide sales. I have not seen specific data yet for Windows Phone 8 but I have not seen much of anything in the news stating that Windows Phone 8 devices are being sold in high volumes. http://www.gartner.com/it/page.jsp?id=2237315
Per Gartner's data linked above, Android and iOS are 86.3% of 3rd Quarter 2012 sales. If you had limited resources and have to pay developers to develop software are you going to develop for the operating system with 72.4% of sales or 2.4% of sales?
Until Windows Phone 8 has a significant market share (I am thinking >10%) Waze shouldn't even consider it. Starting with Waze 3, the Waze application is developed as a native app to both the Android and iOS platforms. It's no longer some common code that is ported to each platform.