iPhoneIndiaBlog.com

Apple has published a support article which cautioned  users about jailbreaking the iPhone. Apples ends this support article with a strict warning – It is also important to note that unauthorized modification of the iPhone OS is a violation of the iPhone end-user license agreement and because of this, Apple may deny service for an iPhone or iPod touch that has installed any unauthorized software.

Device and application instability: Frequent and unexpected crashes of the device, crashes and freezes of built-in apps and third-party apps, and loss of data.
Unreliable voice and data: Dropped calls, slow or unreliable data connections, and delayed or inaccurate location data.
Disruption of services: Services such as Visual Voicemail, YouTube, Weather, and Stocks have been disrupted or no longer work on the device. Additionally, third-party apps that use the Apple Push Notification Service have had difficulty receiving notifications or received notifications that were intended for a different hacked device. Other push-based services such as MobileMe and Exchange have experienced problems synchronizing data with their respective servers.
Compromised security: Security compromises have been introduced by these modifications that could allow hackers to steal personal information, damage the device, attack the wireless network, or introduce malware or viruses.
Shortened battery life: The hacked software has caused an accelerated battery drain that shortens the operation of an iPhone or iPod touch on a single battery charge.
Inability to apply future software updates: Some unauthorized modifications have caused damage to the iPhone OS that is not repairable. This can result in the hacked iPhone or iPod touch becoming permanently inoperable when a future Apple-supplied iPhone OS update is installed.

[via - Apple Support]