It's a moment we've been waiting for for a long time. No, Microsoft's various operating systems are not merging quite yet, but they're about to get universal apps that work across all devices.
Universal Windows apps will make use of scaling UI features to (ideally) fit all screens; this is where that uniform UI design play finally starts to pay off in a real way. Details are sparse for the moment, but the concept is promising. A unified experience is what we've been expecting out of Microsoft for a long time, and this is a great way to dip a toe in the water.
It's also a great way to drive desktop users into the Windows Store and towards apps, instead of regular desktop software, by offering up cross-platform functionality as a bonus. This is only the beginning of what's sure to be a long play, but the road ahead is pretty clear.
Update (18:41): Universal apps aren't just coming to phones and desktops; they'll also be coming to the Xbox One.