A special edition of Nate True's iBrickr software will prepare the image used for the update and a second installer portion will flash the phone. Smxy repository maintainer, Shaun Erickson has packed up this second portion of the jailbreak, which will be available via Installer.app." Via TUAW .
I'm doing this to my iPhone currently and will post my results.
UPDATE
iBrickr has just checked my iPhone, and is now downloading the 1.1.3 firmware from Apple.
Applying the 1.1.3 soft upgrade via Installer.
The 1.1.3 soft upgrade was applied, iPhone rebooted. iBrickr is uploading.
****UPDATE****
I and many others are finding this update isn't working. I'm still holding a Jailbroken 1.1.2 iPhone.
Firmware 1.1.3 changes where and how applications are installed, people who have gotten this to work can't access their 3rd party apps yet. I'd recommend holding off on this until things gets sorted out.