Two things to note here.
The first is that Symbian is a microkernel. Security for a microkernel is much better in general. I haven't seen any major security problems with the KERNEL (I'm not denying their existence and would love to see an example of one).
Symbian uses a POSIX-based security model. The only malware I can find for Symbian is malware that MUST ASK the user for permission to be installation. Users installing unsafe applications is neither a kernel problem nor an OS problem. Problems such as modified firmware still require physical access to the device and time for flashing (note: I don't view software signature "problems" as a problem as I believe that flashing should be allowed even if it does pose certain risks).