Windows 10 for Phones is likely a year out, but it seems some testing is already going on in the wild.
Chinese site ITHome.com reports that a new version of the OS has started showing up on their server logs, running various builds Windows Phone 8.15, which has previously been identified by ex-Microsoftie Nawzil as Windows 10 for phones.
Given that the latest version of WP8.1 GDR2 on the Developer Preview is 8.10.14203, we can only speculate what features are in the 8.15.12449 branch.
Hopefully it will make it to devices, even in Developer Preview, sooner rather than later.
Thanks D H Dog for the tip.