well that the iphone/touch/ipad can only run one application at a time is misleading. apple only allows the user to run one foreground application at a time. the OS itself is a unix-based OS and it actually is running several things constantly in the background, including mail checking, cleanup tasks, phone support, SMS support, and more system-related things. any smart virus is going to insert itself as a background task (known as a daemon process in unix speak) and not require the user to launch it from an icon to cause damage.
the advantage the iDevice OS has over other computers is that it is a very limited operating system. it doesn't run with open "ports" that allow most viruses to connect to it over a network and break in. also, since it is running unix it has a good set of access controls built in, a virus type application would have to have superuser (root) privilege access to be able to do anything it wants. getting this elevated access on a non-jailbroken phone is pretty much impossible at this point, which is why we haven't seen any viruses for the iDevices. there have been some security holes discovered in the past (one involved a buffer overflow in the SMS application) but Apple has moved quickly to patch them and will probably continue to do so.
from a security standpoint the iDevice OS is somewhat decent. not having flash on there helps too.
