Up until July 11 (that's this Saturday, folks), you can preorder upgrade versions of Windows 7 Home Premium and Professional at reduced rates, but just because you can already buy the next-gen OS doesn't mean it's finished yet. It is, however, getting very close.
Several sources are reporting that Microsoft has confirmed Windows 7 will reach RTM (release to manufacturing) on July 13, two days after the preorder pricing ends. This is significant because once Windows 7 goes RTM, Microsoft can then distribute final versions to computer manufacturers in anticipation of the October 22nd release date.
In the meantime, you can still download the polished Windows 7 RC (release candidate) and run the OS until March 1, 2010. Once March 1st of next year rolls around, PCs running the RC will shut down every two hours before completely expiring on June 1, 2010.