From Gartner's report on Q3 2010 sales of smart phones, rendered by Wikipedia above, Symbian is still the king of the hill with 36.6% of the OS share. However, they have taken a sharp hit year-by-year, down from 44.6% they year before. Meanwhile, Android increased drastically, from 3.5% to 25.5%. All the other smart phone OSes decreased, with Windows Mobile on par with Linux and "other", at 1-3%.
Symbian will not reign much longer though, since Nokia is moving to the MeeGo OS for their next smart phones. Since iPhone 4 was launched end of 2010, it seems fair to expect a boost in Apple's share. Several high end Android based phones have also been launched lately, so a further most might be expected here has well.