Due to the structure of the mobile communications system, cellular phones are significantly more vulnerable to "cellular hacks" than other types of phones.
The two major dangers with mobile communications are eavesdropping, and rebilling.
On analog networks (which, as of late, have been replaced with the two types of digital networks: CDMA and GSM) anyone within range with a scanner could pick up your cellular signal, and listen to what you are discussing. With the recent move towards CDMA and GSM digital networks, calls are encrypted for your protection, though, eavesdroppers with the right patience can still decrypt these calls, and listen in on your conversation.
Protecting one's self against eavesdropping is quite simple – consider your cellular phone as a handheld radio broadcasting station (as, that is effectively what it is) and do not say anything confidential. It's unsafe practice to blindly trust any communication medium, and always important to remember that neither cellular communications, nor landline communications are infinitely secure – both have hundreds of faults and potential exploits which should be considered.
The second type of cellular fraud is called Rebilling, or fraudulent billing. When a criminal gets a hold of your cell phone signal, they can dissect it to steal, and clone the phone's identification numbers – the ESN and the MIN. Once this information has been stolen, they can program it in to their own phones, and start billing straight to your account. This type of cellular criminal is referred to as a 'cloner,' as they clone the identification numbers.
Protecting yourself against Rebilling is slightly more difficult than eavesdropping. The cellular networks have done a lot to increase the difficulty of the cloner's job, however, cloning is still happening. Disabling "roaming" support on your cell phone is one easy way to eliminate much of the risk of cloning, cloners like roaming-enabled phones, as it is difficult for cellular networks to track roaming-enabled phones and shut them down for fraudulent use. Other than limiting your roaming usage, protecting yourself against rebilling is likely a lost cause – instead, you should ensure you review all your cellular bills, and report calls to your service provider.