I thought I would start a thread regarding X1 info that is shared at Build.
Day one didn't show much, but they did have a demo of Project Spark and started the beta sign up for that.
The Day 2 keynote has been more interesting for new info. During their many demos, they showed off an app that was running via windows 8 and then pushed that same app to the X1. They pointed out that since the X1 shares windows 8 core, all of these apps are possible and showed how easy it is for devs to work on apps going to the X1 via Visual Studio.
They summed up the conversation by saying that if you want to get a head start developing apps for the X1, then create a windows 8 app.
They also mentioned the Unity engine and how it is supported on the X1 so that game devs could build a single game using Unity and push that across a windows 8 pc or the X1.
Pretty interesting stuff that could lead to a real advantage for the platform. What we still are waiting to hear is how the store will work for the X1 and what guidelines devs have to follow to publish to the X1.