UPDATED: Do you know what the leading mobile OS was in 2005? According to a new report from research firm The Diffusion Group (TDG), Symbian's once ironclad grip on the mobile space is slipping and soon expected to yield its title in the fiercely competitive mobile OS arena. Symbian held a 51 percent market share at the end of 2005, down from 56 percent in 2004. Linux came in second at 23 percent,