Very interested to see how Apple and Google handle the plans to support iOS and APK apps in Windows 10. The fact it will be capable of running these mobile apps is great. How do they plan on getting these apps installed is another matter. Will they somehow interface with the Apple and Google Play stores?
They're not talking about running non-native apps. Blackberry tried that and it was less than optimal. They're talking about making it super easy for devs to port existing code, by supporting it in the new version of Visual Studio. The devs would have to do relatively minimal work to get their iOS or Android code compiled and running on Win10 platforms (including Phone) since it would require few changes.
This is the ideal solution since it means the code is precompiled and it's basically a native app - though a straight port might not be fully optimized. Obviously anything relying on outside code (games using licensed engines) would require a port of that, or a replacement for it. But if there's demand that will happen too.