Don't get too excited (yet), since this isn't a public release, but an application entitled 'Pebble Notifications' has appeared in the Windows Phone Store, dated from a few days ago, with an internal Microsoft team as the developer and with a reference to an intranet post by way of explanation. Details are therefore sketchy, but I can't see why this wouldn't work if made public at some point. Certainly there are a fair few Pebble smartwatch owners around the world who would love the opportunity of exploring how well their gadgets could work with Windows Phone.

From the Store entry:

Pebble application for Windows Phone. This is an internal test application to get feedback on Accessory Ecosystem Support feature in WP Blue GDR1.

Windows Phone 'Blue' is roughly equivalent to 8.1 and is an old code name, but the latest update to this internal test application is much more recent, indicating some activity on the Pebble front. 

There are screenshots too:

Great to see this happening at long last - we'd got wind that Pebble had been looking into Windows Phone compatibility ages ago, but Pebble is too small and with limited manpower - better that Microsoft tackles this from the platform end, really.

Allowing the Pebble system access to notifications by application and, right, by type. 

It all seems comprehensive to me - come on Microsoft, release this as a beta and let us loose on it!

It should be noted that there are several 'Pebble' utilities from third parties in the Store, but they're all hampered by having to remain in the foreground, which is totally impractical in the real world. Which is why an official Microsoft solution, tied closely into the OS, with greater capabilities, and which works fully in the background, is a much better solution.