You have probably noticed that Microsoft started distributing Windows Phone 8.1 preview SDK for VS2013 last week. It's currently in closed beta, where I am s well, but it has leaked to Internet anyway.
Unfortunately the latest ReSharper is really not working at all when developing WP8.1 apps - XAML support is completely broken and C# intellisense sometimes does not get even initialized. I had to disable R# completely, beacuse it was causing more troubles to me than actually helping.
If possible, please make the support for WP8.1 priority, thanks!