Microsoft released Windows Phone 8.1 to those who enrolled in the developer preview program (i.e., everyone).
Ars’ Peter Bright in his review of 8.1:
The result feels a whole lot more mature and a whole lot more capable than its predecessor. The 0.1 version bump, chosen to align the phone platform with its desktop sibling, belies the true nature of this upgrade. It is substantial, and makes Windows Phone tremendously better.
We might still wish that there were a few more apps, and that developers spoke of the platform in the same breath as iOS and Android, but even in spite of this, Windows Phone 8.1 is a polished, fun, clever, and personal smartphone platform that’s just about everyone can enjoy. It’s a magnificent smartphone platform.
I’ve been using it since earlier today, and the notification centre (finally) alone is more than enough to make this a fantastic update. Sadly, my HTC 8X does not seem to be supported by Cortana – other 8X owners are reporting the same, as do 8X owners on Twitter – which makes me worry a little about Cortana, perhaps, being an exclusive feature for Nokia phones, or it having some other restrictive limitations. That, honestly, would be a shame.
Update: Here’s an 8X with Cortana working just fine, so the original worries clearly aren’t necessary.