Charlie Miller, a scientist specialized in network security identified a vulnerability in the iPhone SMS module. It could create an exploit by sending a specific SMS to an iPhone. One could in theory be able to run a script which could for example force the device to send its geo-positioning parameters, or to activate the microphone etc scary, but still based on his claims.
The scientist already contacted Apple, and will not unveil the details of the exploit taking advantage of this vulnerability before the Black Hat USA expo to take place in Las Vegas on August 2nd.