The Windows 8 Release Preview has been released.  You can download it from here.  If you are prompted for a product key, use TK8TP-9JN6P-7X7WW-RFFTV-B7QPF.  To do Windows 8 Metro development, you will also need to download the Visual Studio 2012 Release Candidate. 

Here are the top links that you need to read to get up to speed:

• "Building Windows 8" blog post announcing the release
• "Windows 8 app developer" blog post detailing what’s changed for app developers since the Consumer Preview and how to get your existing apps up and running on the Release Preview
• “Windows Store for developers" blog post illustrating the changes to the Windows Store with the Release Preview
• Migration Guide documenting most of the API changes that were made from the Consumer Preview to the Release Preview 
• Windows Dev Center containing hundreds of new and updated design resources, technical articles, and samples for Metro style app development
• Windows 8 Release Preview FAQ answering questions on installing, upgrading, using the RP, apps, and IE10

Developers in large enterprise companies may also want to check out how to plan for Windows 8 and the Windows deployment guide.