You didn't give us much background on why you want this feature...
Plus if you really want this and you've seen it on other sites you should do what your trying to prevent :p
Really thou I have to agree with Felgall, additionally your only going to slow someone down who wants your pages code or content.
In the past I've had entire sites content "stolen" from me however it didn't put me into a rampage of trying stop it.
I also suggest checking out the link in Felgall's signature (that red line of text), very decent read on the subject.