You can download SharePoint Foundation 2010 now.
The download page says it works on Windows 7 Enterprise; Windows 7 Professional; Windows 7 Ultimate; Windows Server 2008 R2; Windows Server 2008 Service Pack 2; Windows Vista Service Pack 2.
If you try on Windows 7 or Windows Vista you get this error dialog.
Setup is unable to proceed due to the following error(s):
This product requires Windows Server 2008 Service Pack 2 or above.
Correct the issue(s) listed above and re-run setup.
There is an article on MSDN which describes installing SharePoint 2010 on Windows 7 that I helped write. It is now up to date for the final release of SharePoint 2010.
The reason for the error is that SharePoint 2010 is only intended for use on Windows 7 and Windows Vista by developers. Production uses should use Windows Server,