As noted by The Loop and in a brief announcement on Apple's site, the program has now gone live.
Businesses in the United States can now enroll in the Volume Purchase Program and purchase your apps in volume for distribution within their organization. You can also offer custom apps that provide tailored solutions to customers who have a Volume Purchase Program account.The program supports not only standard App Store releases but also custom B2B apps. Apps custom-built for a company by outside developers or other businesses can be purchased privately through the new volume purchasing program.
Full details on how to participate in the program are available in a guide for businesses (PDF).