If you have an iPhone 3G, the only way to downgrade is if you have a previously-jailbroken iPhone and clicked the 'make my life easier' option on the main screen of the Cydia package management application between Sunday, Sept. 6 and Tuesday, Sept. 8. If this is the case, I would be glad to explain how you can downgrade. If it is not, then no method is currently known to downgrade your phone.