Trying out the pre-release OS requires signing up to join the Windows Insider Program, which... is actually a pretty painless process, so long as you already have a Microsoft Account. All I had to do was agree to some terms and conditions, and I was immediately given a product key and directed to the ISO download links. (For what it's worth, the x64 U.S. English download weighs in at 4.1GB.)
You can watch the video introduction to the Technical Preview here, in case you missed it yesterday. Microsoft has also posted a list of frequently asked questions, in which it warns that the software is a "long way from done," and users should brace for frequent updates and major user-interface changes over time. The company also advises against installing the Technical Preview on a production PC, since crashes and stability issues could "damage or even delete your files."
Oh, and the Windows Technical Preview works best with a mouse and keyboard. The touch interface is functional, Microsoft says, but "some things will be rough and unfinished." I guess this thing really is being developed with desktop users in mind.