As I have heard, symbian is popular (larger than the rest mobile OS together), but in decline.
Since version 9 (or so) all applications have to be signed, so applications will either be from trusted source, or will have to go through computer for signing (where those could be checked by computers antivirus). Anyway, i have never encountered viruses on symbian (used nokia n70 for 3 years and e52 for half year).
This method would not work with "app stores", where only way to get app into device is download from specific centralized mobile formatted website (app store).
So, any desktop computer antivirus can check mobile (symbian, android, etc) apps for viruses?