My understanding is that the iPhone contains a full-blown version of MacOS X. If that is true, then there is a firewall (ipfw) which Apple has locked down by default. This would make it as secure as any other handheld device out there today.

Re: Gartner to IT: Avoid Apple's iPhone.

Also:
Is it not the job of IT (and developers) to not dictate whether a device is useful to an enterprise, but "ask" what devices/tools an enterprise needs and find some way to make them work securely in the environment? Seems a bit "cheeky" of Gartner to be so presumptuous to assume what "we" will find useful.