From the company's blog post:
We want to apologize for the delays in getting Rifts to doorsteps. We appreciate that without your support, VR wouldn't be where it is today. Thank you for your patience over the past few months.
We've also started to ramp up inventory for retail partners online and in stores, and added more Rift demos at new retail locations across the US. Starting this month, you'll be able to get your hands on Rift at select Microsoft Stores. In addition, our demo space will expand at Best Buy to more than 500 US stores now through the fall.