Since installing Safari 3.0, I've found the confirmation boxes for unsubmitted forms to be quite annoying. Now that Web 2.0 is so popular and so many sites send content via AJAX, having unsubmitted forms is not uncommon, and these confirmations are just a pain. The fix? After digging around with...
strings /Applications/ found the DebugConfirmUnsubmittedFormText preference. To disable the confirmations, quit Safari, open up Terminal, and type this command:
defaults write DebugConfirmTossingUnsubmittedFormText 0To re-enable confirmations, just do the same with a 1 instead of a 0 at the end. Enjoy your much-improved Safari!

Props to MACOSX Hints