It depends on whether you jailbreak them, or download apps from non-secure sources. If neither of these is true, then you likely do not require any sort of anti-virus for an iOS device. The doomsday scenario is that someone in your organization has a jailbroken phone that gets infected with a virus that then infiltrates the network from that device, but even then, that virus would be trying to get at corporate data on windows machines, not your iOS device.
Here is visual a layout of iOS security implementation - the mobile iOS is really spartan, and there isn't a whole lot of wiggle room for a virus to get in there unless you open the door for it by modifying your device.
