There was this screenshot of an internal Sprint document floating around that shows an OS 6 update was imminent. OS 18.104.22.1685 was scheduled to begin rolling out over the air (OTA) to users of the BlackBerry Bold 9650 and Curve 3G on December 22nd. Many users are now reporting to get the update notification.
If the update does not get pushed to your device, make sure you click the Options icon > Click Advanced Options > Click Wireless Update. Let us know how the new official OS runs!