A Photo taken during the Nokia Developers Day in Mexico has revealed the two upcoming version of the Symbian operating system after Belle. These updates are named Symbian Carla and Symbian Donna.So it seams that Nokia will be following the alphabet in update names for their Symbian OS. Which started with Symbian Anna which was released in April 2011 followed by Symbian Belle which will be available to all Symbian^3 devices by the fourth quarter of 2011.
Symbian Carla will bring a version 8.0 of the web browser, Richer widgets, Dolby Surround sound support and enhanced NFC functionality.Symbian Donna is said to bring dual-core CPU support to the Symbian platform. Not much more is known about this update at this time, but information should become available in the coming months.
source - AllAboutPhones