OS 5.0.0.592 for the BlackBerry Curve 8520 has been leaked and can be found online. There is no word on what is contained within 5.0.0.592, however, you can download it and find out for yourself if you are feeling adventurous.
Download OS 5.0.0.592 for the BlackBerry Curve 8520 -- Password: BBLEAKS.COM
[Via BBLeaks]
NOTE: This is a LEAKED BlackBerry OS and is not supported. Make sure you understand how to install and, for goodness sake, make sure you have good back up of your device before you attempt to install!