Tomorrow, Microsoft will stage an event where it will pull back the curtain on Windows 10's "consumer experience." According to Microsoft reporter Mary Jo Foley, we can expect a preview of a Windows 10 "mobile SKU" at the event—but we won't be able to get our hands on that version of the OS until next month.
Foley quotes "sources close to Microsoft" whom she says have a solid track record. (Considering Foley's own record, I'd say the info is likely true.) The sources say the mobile version of Windows 10 will run on both phones and smaller tablets, and it will support both Intel and ARM processors. Sounds to me like a combined successor to Windows Phone and Windows RT.
The first "public release" of Windows 10 mobile "won't happen until some time in February 2015," according to Foley. That statement presumably refers to the first public beta, since according to Microsoft, the completed version of Windows 10 is scheduled for a somewhat nebulous "late summer and early fall" time frame. (Thanks to Neowin for the link.)