UPDATE: A downgrade for 1.1.3 is now available. More information can be found here.
Original article follows:
For those of you who decided to try out the latest iPhone firmware in order to check out the new features, planning only to downgrade back to 1.1.2 or 1.1.1 after deciding whether or not the loss of third party applications was worth it, a bit of bad news has found (or shortly will) its way to your doorstep. As it turns out, what was previously witnessed with the leaked 1.1.3 firmware, holds true with Apple's official release - there is no way to downgrade to an earlier firmware version.As mentioned, for the privileged few who got their hands on the leaked version of 1.1.3, the inability to downgrade firmware was witnessed a couple of weeks ago. Noted iPhone hacker Nate True posted on his blog on creations.net that the downgrade
bricked the iPhone, stating
downgrading from 1.1.3 will leave you bricked until you upgrade back to 1.1.3, as far as I can tell. The phone (even with an authentic AT&T SIM) reports an Incorrect SIM while iTunes says there's no SIM at all, when you've downgraded back to 1.1.1 or 1.1.2 (1.0.x untested).
This also holds true for the officially released version of 1.1.3. This should come as no surprise as reports indicate that the leaked version and the official version are the same. Not almost the same, 100% identical.
Newest iPhone FAQs
So, if you haven't yet upgraded to 1.1.3 and want the flexibility to restore older versions of the iPhone firmware - wait. Otherwise, you'll be out of the third-party applications game until the dev team or someone else figures out how to properly downgrade.