Valve and HTC will be offering signups for their Vive dev kits soon.
The bad news; they will definitely have limited slots. The good news, the kits will be free. The bad news (again): the kits will apparently be free ‘for now’, implying you may not want to sign up if you can’t spring for the costs for it down the line.
A select group of devs already have their hands on the Vive headsets, so Valve is working to slowly expand the number of users.