I dont know when but apparently Microsoft Research has updated the Menlo pages to reveal the work about porting NT to mobile devices and replacing CE.

Menlo
http://research.microsoft.com/en-us/projects/menlo/

The Menlo Project explored new user experiences for phones.  We prototyped new hardware, new software platforms, new user interfaces, and new applications. Most of our work instrumental in the creation of Windows Phone 8 and Windows RT for ARM Tablets, but was unpublished.

Experiment 19
http://research.microsoft.com/en-us/projects/experiment19/

A skunkworks project in 2008/2009 to re-imagine the OS platform for Windows Phone. The prototype proved that Windows NT and the CLR could deliver better performance than Windows CE and the .NET Compact Framework on identical hardware. Within months of the completion of Experiment 19, Microsoft launched efforts to build what would become Windows Phone 8 and Windows RT for ARM tablets.

Video shows that NT performs better than CE on same hardware
http://research.microsoft.com/apps/video/default.aspx?id=167896

This video shows a comparison of two identical prototype phones with NVIDIA Tegra APX2500 processors. The phone on the left is running the Windows CE kernel, the same OS kernel used in Windows Phone 7. The phone on the left is running the Windows NT kernel, the same OS kernel used in Windows Phone 8. Dubbed "Experiment 19", the prototype system on the right proved that the Windows NT system could achieve better performance than Windows CE on identical hardware. The system was first demonstrated at MSR TechFest 2009. Filmed in 2009, this demonstration proved for the first time that Microsoft could use the same core windows components for both PCs and phones. On June 20, 2012, nearly 3 years after this video was recorded, Microsoft publicly announced that Windows Phone 8 would use the same Windows core as Windows 8.