The download page for the CDF player http://www.wolfram.com/cdf-player/, only shows version 9.0.1, which shows an error message when I open CDFs created with V10.
CDF Player 10 was finally released recently:
Its also worth noting that CDF Player 10 has had 1 update since release with no version number change - so if you downloaded it in the first week or so you should probably re-download. On a windows machine there is about ~3k difference in file size. This was due to issues with the plugin when using Chrome and Firefox. On my system the file sizes were 589,746KB (older) vs 589,743KB (newer). Note your file sizes may not be the same due to differences in FileSystem and OS.