Apple recently released an iPhone 3GS that was supposedly resistant to jailbreaking. Well, it's already been cracked according to Gizmodo. The crack is not perfect as you have to have your iPhone tethered to your computer to boot up, but it surely won't be this way for very long.
"It turns out the new boot ROM doesn't totally prevent the 24kpwn exploit employed by the Dev Team hackers, but instead just kind of interferes with it. The result? You can still jailbreak your late-model 3GS, but the device needs to be tethered to your computer in order to boot up. It's a major annoyance, especially given how crash-happy 3.1 phones—especially jailbroken ones—can be, but not necessarily a dealbreaker."
"It turns out the new boot ROM doesn't totally prevent the 24kpwn exploit employed by the Dev Team hackers, but instead just kind of interferes with it. The result? You can still jailbreak your late-model 3GS, but the device needs to be tethered to your computer in order to boot up. It's a major annoyance, especially given how crash-happy 3.1 phones—especially jailbroken ones—can be, but not necessarily a dealbreaker."