It isn't exactly official, but if reports based on a leaked internal video are true, the Windows Phone 8 (WP8) codenamed Apollo is going to be based on Windows 8 code.
This is perfectly reasonable and to be honest if Microsoft did anything else there would be an outcry about fragmentation. However, in this case it isn't possible to please everyone.
The basic details of WP8 are that it will support multicore processors - and it might well have no choice given the bloat involved in the systems software - higher screen resolutions, support for NFC (Near Field Communications), improved data tracking, native bitlocker encryption, secure boot and a Skype app.
It will also feature IE 10, but with a proxy server feature aimed at speeding up web access. This will make IE10 on WP8 more like Amazon's Silk browser with all web browsing passing through Microsoft servers. What isn't clear is whether WP8's IE10 will be based on the WinRT version, which doesn't support plugins like Flash, or on the Desktop version, which does. Given the overall direction that Microsoft is taking, my bet would be on the non-plugin, pure HTML5, browser..
The technical details are that WP8 is going to use the core of Windows 8 including the kernel, networking, security and multimedia support. It also seems that the system will run all WP7 apps and this means that it will support Silverlight. However, it is also going to support "native" apps written in C++ in addition to Silverlight apps. This sounds like WinRT apps are going to be supported on WP8 and if this isn't the case then Microsoft has just invented yet another way to code apps for a Windows 8 like system. Given that apps are also going to support various WinRT features like contracts, it seems that WinRT apps and Silverlight apps are going to coexist.
So what does this all mean?
Given WinRT native apps are flavor of the month at Microsoft, it would be a naive (not native) programmer who didn't realize that this marks the end game for Silverlight. It has been dumped in Windows 8 and now it is made legacy on its only niche survival ground - the Windows Phone. WinRT is going to be the only technology that spans all of Microsoft's platforms - phone, tablet and (sort of) desktop.
The conclusion has to be that you can expect to work on existing Silverlight apps for now, but for any new projects you need to pick WinRT.
In a stroke Microsoft have made 100,000 Windows Phone apps legacy.
It also raises the question of why WinRT on Windows 8 can't support Silverlight for a transitionary period to Windows 9 say? Not doing so just seems to be stamping out Silverlight's existence faster than is actually needed.
Then there is the question of code bloat.
WinRT was touted as being a leaner Windows just right for tablets and presumably as we now know, just right for phones. But including Silverlight support seems to demand not only the new WinRT but the old WPF type framework be included in the system code. How much this inflates the software footprint is difficult to say, but it must be leaner without Silverlight.
Perhaps this is the reason that multicore isn't an optional extra.
It is very clear that dumping Silverlight, and the whole .NET adventure, is a big mistake. It would have been much simpler to base the Windows 8 tablet extensions on Silverlight and consolidate the position rather than throw it all away and start again from scratch.