If you make use of unofficial third-party iPhone applications and would like their presence on your phone to persist, refrain from updating to iPhone software/firmware 1.1.3 when it is released. Early indications are that this update not only disables any previously enacted open reading/writing of the iPhone filesystem (jailbreak) hence disallowing unofficial applications to run, but also makes it -- currently -- impossible to downgrade to previous iterations of the firmware/software that can make use of unofficial applications. As such, updating to firmware/software version 1.1.3 means you'll be waiting for the development of a jailbreak method that works with the new firmware release or an ingenious method for downgrading to a prior release.
Previous iPhone software/firmware updates also temporarily disabled the use of third-party applications, but generally had allowed for easy downgrading to previous iterations that could be jailbroken until a new jailbreak method was discovered. Phones upgraded to firmware version 1.1.3 reportedly become non-functional upon downgrade attempts until they are re-upgraded to 1.1.3.
These data combined with reports that iPhone 1.1.3 will not be any easy nut to crack with regard to jailbreaking should persuade users who hold unofficial third-party applications near and dear to hold off on the update (in spite of its significant new functionality) until further notice.
Upgrading to iPhone version 1.1.3 (or some subsequent release) will almost undoubtedly be required for use of official third-party applications sanctioned by Apple and built with the forthcoming SDK.