Check out the MSDN Magazine article by Microsoft Press author, Andrew Whitechapel, on building an app for Windows Phone and iOS.
MSDN Magazine article: http://msdn.microsoft.com/en-us/magazine/jj658972.aspx
There’s also an interesting follow up to his article here: http://blogs.msdn.com/b/msdnmagazine/archive/2012/10/19/10361207.aspx
Right now, you can order the ebook version of the upcoming book, Windows Phone 8 Development Internals, on our distributor’s site, oreilly.com. You will receive versions of the book as it’s being written. Click through to see the details of the offer: http://shop.oreilly.com/product/0790145373342.do?sortby=publicationDate#
You would think Andrew would have used MonoTouch for the iOS version and shared code between both versions. Here's a much better cross-platform article covering Windows Phone, iOS and Android and sharing the bulk of the code between them.