Does it look familiar? It is especially annoying with Google Instant because google reloads the results without actually submitting the form and Safari thinks you are about to throw away the input.
To get rid of the prompt open up Terminal and enter the following command:
defaults write com.apple.Safari DebugConfirmTossingUnsubmittedFormText -bool FALSE
Restart safari and reclaim your sanity.
hallelujah! I thought Safari was trying to kill me with this INANE and COMPLETELY useless message! Hey, Apple. If you are going to try to help user in a Microsoftian way, then PLEASE give us the option to get RID of it. It was like trying to drive a Harley with training wheels! Eeek.