I am not sure what this exactly means, but the letter from Satya Nadella mentions,
To win in the higher price tiers, we will focus on breakthrough innovation that expresses and enlivens Microsoft’s digital work and digital life experiences. In addition, we plan to shift select Nokia X product designs to become Lumia products running Windows. This builds on our success in the affordable smartphone space and aligns with our focus on Windows Universal Apps.
This is certainly not such a big news to share as Microsoft can obviously build Lumias with Nokia X design language at will. So, did Satya mean, replacing Nokia X OS with Windows Phone OS in upcoming Nokia X family of phones, or is it about dual-OS Nokia X phones able to run both Android / Windows Phone? It may also mean that Microsoft may be planning to do away with Nokia X family of devices soon.