The iPhone Dev-Team has posted a brief note warning iPhone jailbreakers and unlockers to avoid updating to firmware 3.1. The concern comes as expectations grow that Apple may release the 3.1 firmware at their September 9 2009 iPod event.
If the 3.1 firmware update is released on Wednesday, jailbreakers and unlockers must avoid updating to it otherwise those iPhones will lose their ultrasn0w unlock, possibly permanently.
This warning is mostly for users that use redsn0w, as the Dev-Team has indicated that their PwnageTool will allow you to update the firmware without affecting the baseband.
But in any case, if you have a jailbroken and/or unlocked iPod Touch/iPhone, it would be wise to avoid the 3.1 firmware update until further notice.