Currently, people who sign up for the Windows Insider Program get access to the new preview builds of Windows 10 itself. Microsoft announced on Wednesday that 1.7 million people have already registered with the program. In today's blog post to developers, Microsoft stated "if you want to be one of the first ones to receive early previews of our tools please sign up today." There's no word on when they plan to offer those dev tools in the program.
Most of the blog post repeated what Microsoft has already revealed to developers about its plans for Windows 10, along with links to some online documentation and training to help them get up to speed on making apps for the OS. It added:
"We're more committed than ever to making sure that you can leverage your work to reach more customers, regardless of where they are, what device type they're on, or what operating system they're running. The best way to start preparing for Windows 10 is to start building universal Windows apps today for Windows 8.1."