Con: Beware of the Rickroll
Unless you're a big Rick Astley fan, you're probably not going to want him plastered on your jailbroken iPhone. Jailbroken iPhones in Australia were reportedly infected by a worm that only replaces the device's background wallpaper with an image of 80s one-hit wonder Rick Astley. The worm, dubbed ikee, only affects jailbroken iPhones running the SSH app with the default password. While the worm is fairly harmless and seems to only be limited to jailbroken phones in Australia, malicious variants of ikee could soon follow.
Con: Bye-Bye, Warranty
Sad, but true: Jailbreaking voids your warranty. If you experience hardware problems with your jailbroken iPhone, don't expect any help from Apple. But you can probably get around that limitation. If you use iTunes to restore a clean version of the iPhone software (not one from a backup), Apple will never know you had previously jailbroken your phone.
Con: Updates Can Break Jailbreaks
If you're the kind of person who waits in line to buy the latest and greatest Apple hardware, or you worry about the security implications of not installing a new software update, jailbreaking may not be for you. If you install an update on your hacked iPhone, it will likely negate your jailbreak, meaning your phone will still function normally, but all your jailbreak apps and plug-ins will be erased. Although the Dev-Team generally puts out new tools within a week or two after new software is released, you won't be rocking the shiny new features on launch date.
Con: Increased Instability
Every operating system is prone to crashing every once in a while, and the iPhone OS is no exception. Throw some unofficial, hacker-originated software into the mix and you might experience more crashes. Also, any add-on or UI mod you install could slow down your iPhone.
Con: Security-Breach Risk
Jailbreaking may open up root file-system access, which makes it possible for users to unwittingly do serious damage. For example, if you decide to install openSSH (to allow wireless data transfer between your phone and your computer) and forget to change the default root password, your entire file system could be open to others on the same network, and your personal data could be at risk.
Con: Fear of the Brick
While the Dev-Team has generally done an adequate job of testing its tools prior to release, cases of users "bricking" their iPhones in the jailbreak process have been known to occur. The jailbreaking tools are as user-friendly and safe as they have ever been, but you should realize that jailbreaking comes with some inherent risks. If you love your iPhone and don't want to gamble the hundreds of dollars it may cost to replace it, jailbreaking may not be your best option.