Let me just add that the betas are numbered 0.6.9x while the normal releases are numbered 0.6.5x
On windows if you try the beta and want to go back to the normal releases, first uninstall calibre then install the normal release. And remember not to do this with the same library.
On linux do the following (assuming you are using the calibre binary from the calibre website)
Code:
cd /opt/calibre
tar xvjf /path/to/calibre-0.6.90-i686.tar.bz2
This will replace your current calibre install with the 0.6.90
And a big round of applause to chaley for getting this done