Not the thread - the phone seams to be misleading.
Did the exact same test the following night. Phone charged to 100%. Disabled all running programs. Received 3 E-Mails that night. This time I did the test also with my 1-year old 3GS as a comparison. The 3GS was synced with the same backup as the 4.
This time my iPhone 4 showed 100% battery after 9h of standby (88% on the first test). My 3GS had only 93% battery left.