This is probably posted in various threads but I thought I'd dedicate a thread to it in the hopes it will warn those who want to try 1.1.3 and then go back to a jailbroken 1.1.2/1.1.1.
As of this moment: IT CAN'T BE DONE, at least not if you still want to be able to use your iPhone as a phone, and for EDGE data.
Again: If you update to 1.1.3 right now you will be stuck with it. And as of right now 1.1.3 cannot be jailbroken.
Update: It is now possible to downgrade from 1.1.3 if, and only if, your bootloader is version 3.9 (OOB iPhone versions 1.1.1 and earlier). It's a lengthy process but does work:
http://www.modmyifone.com/forums/showthread.php?t=25695
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Mike