What if the Windows Phone 8 secret is Dalvik?
Of course, this is simply idle speculation, and apologies if this has been discussed before. But what if the secret that Microsoft was keeping close to the chest about Windows Phone 8 is that they have created a Dalvik VM for Windows Phone? This is at least theoretically possible; the Dalvik VM is open-sourced under the Apache license. It would also explain the secrecy: Microsoft would not want to undermine its own efforts to promote WP8 native development.
If my understanding is correct, this would give Windows Phone the ability to run Android apps that do not use native code, which would significantly increase the breadth of the Windows Phone app library, one of its points of criticism. Effectively, it would piggyback off of the alread-successful Android ecosystem.
It would also be able to claim to be the most versatile mobile OS, in terms of being able run apps from multiple ecosystems.
Developers would be able to target two ecosystems at the same time, increasing devloper interest in WP8. This could be a part of a larger EEE strategy: first make it easy to develop for WP8 alongside Android; then make it even easier to develop than Android; then make introduce additional features that Android doesn't have.
Clearly, the biggest disadvantage would be undermining the WP8 ecosystem. By being able to develop for Android and WP8 at the same time, developers have little incentive to use the WP8 API and produce apps that are exclusive to WP8, which are very desirable to promote a platform.
However, there would still be the issue of native code. Native code apps for WP8 - which we know will be available - won't use the VM and thus will have to use the WP8 SDK. This would make native-code WP8 apps exclusive to Windows Phone. Since native code will be required for the best performance and fullest access to the API, this could be enough of an incentive to get some top-shelf exclusive apps.
A side effect of the above disadvantage is that Microsoft would be indirectly helping one of its most formidable competitors. By making Dalvik a desirable development target, it would be giving developers an incentive to create great apps for Android too. Since Microsoft is competing against Android, and Android is far ahead, this doesn't help Microsoft.
On the other hand, Micrsoft has little to lose at this point, and even though they would be encouraging the development of Android-compatible apps, they would also be discouraging the development of Android native-code apps, which would undermine Android's ecosystem adantage.
I don't think it's likely that this is the big WP8 secret, if there is a big WP8 secret. The biggest reason it's not likely is that I wouldn't expect this to be much of a factor in the SDK, so it wouldn't explain why the SDK is being kept under such tight wraps (though I could be wrong about this.) But it would certainly be a huge shakeup if Microsoft were to do this, and it's fun to specualate as to how it would shake up the existing ecosystem dynamics.