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Their may eventually be a release on a way to do this but as for now its safe to assume that the answer is no.

Also its questionable if their will ever be a way to downgrade with the SDK release just around the corner installing 3rd party apps on the iPhone should be close behind as an option without jail breaking the phone.
 
if anyone is tempted by 1.1.3, don't do it!! if you want to keep your apps, wait to hear from the dedicated hackers about potential downgrades/jailbreaks.
 
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