Who do you trust: When you jailbreak an iOS device using a tool as Jailbreak Me or one of the tools offered by the Dev Team, you don't really know what's happening to your device. At the end of the jailbreak you should have a more open iPhone, iPod touch, or iPad and a copy of the Cydia app. But what else has gone on in the background?
Up to this point, nothing, it seems. I've jailbroken iPhones and iPod touches for years and I've yet to have a problem where my data was exploited. But, as they say in the questionable-investment business, past performance is no guarantee of future results. It's possible that a jailbreak will come along (though not likely from the iPhone Dev Team) that does The Bad Thing.
Your phone is more open to attack: When you jailbreak your device and enable SSH, its more accessible from the outside than it once was. Far too many people jailbreak their phones (or purchase jailbroken and unlocked phones) without then changing its default "alpine" password. And some of those people have suffered in the form of baddies breaking into their devices.
If you're going to jailbreak your iPhone, do your future self the very big favor of changing its password. Cydia's home screen provides a Root Password How-To button. Tap it to learn how to change the device's password.
