Joe Belfiore said Windows 10 dev preview for Windows Phones in 2015.

In reply to a question, Joe Belfiore has said on Twitter that Windows 10 developers preview for Windows Phones and Windows RT Tablets is not going to happen before 2015. This is in line with what we reported via our sources. 

The Windows 10 for phones may bring actionable notifications and split-screen multitasking for Phablets amongst new features.