Android, with its open nature being in the regrettable position of leading the pack when it comes to malware threats isn't much of a surprise. Symbian being second somewhat is.
A US department of Homeland Security report cites F-Secure figures that suggests that 79 per cent of mobile malware rests in the Android ecosystem; not surprisingly the strategy favoured by the DHS would be to install a security suite, but probably not one that they themselves couldn't crack.
Symbian is the next most targeted mobile OS, which I suppose shows just how long and how hard Nokia had its roots in the feature/smartphone space before switching to Windows Phone 8. Way down the list sits iOS with 0.7 per cent of the threats, the same as "others", although I'm stumped as to what the others might be, given that if you want a threat-free phone, the way to go would appear to be either Blackberry or Windows Mobile, with only 0.3 per cent of the threats detected.