In short, I don't think this can be disabled in Safari. It's a precautionary warning to those who forget that you shouldn't click "Back" after you submit some sort of data (i.e., a shopping order) as it could potentially send the data (or shopping order) again.
I remember a while back being able to disable this warning in Firefox using some config haxies or an extension, but it wasn't easy. Having this warning is standard practice in most, if not all, modern browsers.
In case you need some more info, check this Apple Discussions thread.
http://discussions.apple.com/thread.jspa?threadID=1934998&tstart=75
P.S.
What theme is that? I really like the rectangular buttons, instead of the pill-like buttons in OS X.