I get asked this question a lot. TLDR: Believe me when I tell you, the path to least resistance is to write a game using Unity, and bring it to Windows 8.
Microsoft has partnered with Unity to make the process for going from Windows 8 to Xbox One as seamless as possible, as stated by this official announcement from the ID@Xbox team.
"To us, ID@Xbox is about providing a level playing field for all developers. So, we worked with Unity and we’re pleased to announce that, when released in 2014, the Xbox One add-on for Unity will be available at no cost to all developers in the ID@Xbox program, as will special Xbox One-only Unity Pro seat licenses for Xbox One developers in the ID@Xbox program." - Chris Charla, Director, ID@Xbox
Rather than wait for Microsoft to unlock Xbox One units and turn them into dev kits, just get started on making a Windows 8 game in Unity. Think about it: Your game is immediately more attractive to Microsoft the moment you use an engine that can be ported to three of their platforms: Windows 8, Windows Phone, and Xbox One.
Need a hand? Get in touch with me -- It's my job to help!
Now we're talking my language! Alternatively, you could use HTML5 for apps. (Notice how I didn't say anything about games in there?) From a Gamasutra article in September 2013, Microsoft's EVP of operating systems Terry Myerson stated:
"We want to offer [developers] the opportunity to build either HTML5 applications, or native applications that span all of those devices, enabling them to reach segments of users on those devices, users on a gaming console, and provide them with very unique opportunities to monetize their application investments," he explained as part of today's Microsoft Nokia Transaction Conference Call. And this includes allowing for HTML5 and native applications across all the company's devices, including smartphones, tablets, and the upcoming Xbox One. - Mike Rose, Gamasutra
Maybe C++ is your thing. In that case, you can use the C++ / DirectX Stack to get your game on Xbox One. Chuck Walbourn and Shawn Hargreaves have been doing a phenomonal job of updating this code base each week, and you can download it right now to get started. This is basically the C++ version of XNA, so if you are familiar with XNA, you'll find that it's very similar.
What do you know: I even put together a Power Point presentation that I share with schools, to get students on board as well. Here is a link to that. I update it from time to time as well, so check back on occasion. View the presentation here.
Think of new and innovative ways to take advantage of the tech that Windows 8 and Xbox One offer. What about creating a second screen experience? Something where you take the role of a Dungeon Master in in Dungeons and Dragons using SmartGlass, and your friends are using gamepads on the main screen to control their characters. What if the second screen device was an alternative controller for selecting media and playing it on the Win8 / Xbox device? Think outside of the box and it it will grab Microsoft's attention.
I'll also be on Scott Hanselman's Hanselminutes podcast tomorrow, where I'll be talking more in depth about Xbox One development, so be sure to follow along there as well. How do I know all of this? Well I was writing Xbox One and SmartGlass stuff at Comcast, but I also work at Microsoft. Everything listed above though, is publicly available information, so you're not going to find anything new here; I simply organize it for you. So I thought I'd write down all of the publicly known possibilities.
If you'd like some more information, leave a note below and I'll see what I can do for you.