They just have to be prevented. The success of the whole concept is in the balance. People are not going to accept the same level of problems with their phones as with their desktop. Rebooting phones and worms sending MMS that the customer
have to pay for. Oh brother. Should that happen the smartphone is dead, and people will go back to the basic services (voice and SMS). For the majority that is very much an option.
Not to mention what will happen if someone successfully creates a virus that DoS 911... Some countries have already outlawed camera phones for way less.