Apple has made the next move in their continued struggle against modders and other iPhone enthusiasts, by increasing the iPhone 3GS's resistance to jailbreaking. It's well known that each iteration of iPhone OS (and often the various updates in between) incorporate new measures to stop jailbreaking, and the most recent update is no exception. New iPhones are making their way into the wild with an updated version of iBoot, the bootloader for a variety of Apple devices. Because this software is invoked before the iPhone OS is actually loaded, it is a common target for jailbreaking – and the recent update prevents many of those attempts.
It seems the latest versions of common iPhone jailbreaking tools are unable to get around the most recent update for iBoot, making it impossible (currently) to install software that Apple hasn't authorized. The article mentions that patching jailbreak "holes" could simply be an unfortunate side effect of improving security on the iPhone, which is possible – though it's not like Apple has ever shown any love to people who want to jailbreak or otherwise mod the iPhone.
Even if Apple is not directly targeting jailbreaks with this update of iBoot, they have in the past. Jailbreaking is in high demand for iPhone enthusiasts, so as more of the newly updated phones are circulated, you can be sure that fresh jailbreaking methods will be developed.