Still, better late than never. Microsoft has told Penny Arcade that the delay is because of their overhaul to the underlying technology between the new Xbox One controller and the previous Xbox 360 controller. They have introduced a new wireless protocol (great, more new wireless protocols), combined with the ability to work in wired mode, and the addition of features such as Impulse triggers, means that the new software has been written and 'optimized' for PC as well. Microsoft also has to ensure that current PC games that are built for the Xbox 360 will work with the Xbox One so they are kind of busy and have other priorities.
While you wait for Microsoft to come out with the software update, and Xbox One, you can use your PS3 controller (if you have one) and hook it up to your Bluetooth 2.1 + EDR compatible PC, or buy a $5 dongle if you don't have one. You need to download Motion Joy to make it work though, still cheaper than a Xbox 360 adapter.
Source: Penny Arcade