Wow, been a while since we seen a leaked OS for the BlackBerry Curve 8520 that's for sure. in any event, we now has OS 5.0.0.1022 up for grabs. While we're not sure as of yet what may have changed within the OS, we do know it's unofficial and with that you should be sure to back your data up before loading just in case.
*OS Disclaimer - Please Read: Keep in mind this is NOT an official release and as such is not supported by RIM or your Carrier. If you choose to install, do so at your own risk and only if you're comfortable in the process of upgrading and downgrading your device software. It is quite normal during operating system development for certain features to be crippled or disabled while others are tested, and these beta builds often reflect that. They are not always better. Depending on your BlackBerry comfort level, it may be best for you to wait for community feedback on a build before deciding to download and install for yourself or simply wait for the next official release. If you require support, you will find it in our CrackBerry forums via our extremely knowledgeable community base - do not contact your carrier or RIM for Beta OS technical support (their first course of action will be to advise you to downgrade to a supported OS). Follow the links below for more information and instructions. *
Source: BBLeaks