Save Warning on exit

Any guidance on implementing a feature that would warn a user if they attempt to leave a page that has been edited but not saved?  Some kind of annoying popup:

"You have unsaved changes - discard, save or cancel?"

I figure this might happen in a body unload event, but how to detect whether the editor is "dirty" (unsaved changes) or "clean" (unchanged)?

-- Bob Stein, stein@visibone.com


By the way let me take this chance to say this is the most awesome online rich-text editor I've found!  (Tried before it Nicedit and TinyMCE.)  It was agony to implement this on a L.A.M.P. server but I got it done (by purchasing an ASP host from winhost.com and using iframes and several dirty tricks).  Web Wiz RTE looks good, the ergonomics are tight and mostly flawless (some weirdness with lists and indents but they all have that).  And the stuff just freaking works.  Thanks whoever in the world made this the great quality thing that it is.