The countries covered by the deal are...
Microsoft's acknowledgement that Yandex is a a better option for consumers than Bing in certain territories is a mature and sensible decision, and I'm happy to see this relationship continue. It will be hard-coded into the OS, which is a pty. At the very least I would have loved to be able to switch to Yandex next time I am in the CIS region.
This decision will probably lead to many people wanting to have an option to switch their Windows Phone to another search engine (I can hear the knee-jerk reaction asking for Google in the distance), but it's not as simple as changing a search engine box in a web browser. Search is built in to many levels of the core operating system, and I suspect it takes a fair amount of engineering and testing time to make the switch.
The full report can be found at The Next Web.