That means an App on iOS can not talk to another App, and can only talk to the phone with certian commands (or API's) that Apple allow. An indiviudal App may have a virus, but only if the Developer compiles it, and it only effects that App. One exaple of this was a Scanner App that got a list of your contacts (Which Apple Allows) and sent them to a 3rd party company. Deleting the app removed the 'virus'.
This means 2 things,
1. Its very hard to get a virus unless your phone is jailbroken
2. A Sophos Anti-virus App would not be able to scan other Apps, making it fairly usless
There is already the Sopho Mobile Security App, that looks at what Apps are installed, and will tell you if a App is known to be bad, or have known issues. Like the Scanner App I mentioned earlier.
