Over at the Windows Team Blog it was announced yesterday that RTM will be the first week in August, with general availability slated for the end of October:

For the first time, we provided details on Windows 8 availability. Tami confirmed that Windows 8 is on track to Release to Manufacturing (RTM) the first week of August. For enterprise customers with Software Assurance benefits, they will have full access to Windows 8 bits as early as August. Additionally, she noted that RTM is when we’ll be turning on the commerce platform so that developers can start earning money for their apps – we'll have more to share on the Windows Store for developers blog soon. Of course, right now with the Windows 8 Release Preview, all apps are still free for people to try.

Tami went on to say that Windows 8 will reach general availability by the end of October! This means that new Windows 8 and Windows RT PCs will be available to buy and upgrades will be available starting in October. She also mentioned that that Windows 8 will be available in 109 languages across 231 markets worldwide.