I had the similar problem on my iPhone, after change password using passwd, the Springboard gaves an infinite loop, and when I wanna try to reboot it. The phone is stucked on the appl boot screen.

What I did is boot the iPhone in restore mode, connect it to itunes and let it do the restore. Then using Ziphone to unlock and jailbreak it.

To enter restore mode on iPhone,

1. Hold the sleep/standby button on top + Home button to force it to shutdown
2. Keep holding the Home button and  release the sleep/standby after it shutdown
3. Press again the sleep/standby button to boot it and immediate release it when u saw the apple boot logo
4. At the point, u r require to still holding the home button, wait for around 10-15 secs, the iPhone will prompt u to connect it to itunes


Hope this helps.