10.8.2 brought the option to choose between either saving unsaved changes back to the original document as well or leaving the original document in its last-saved (committed) state when invoking Save As; here are my findings.
If you’ve made changes to the document at hand, a checkbox will appear at the bottom of the dialog: ‘Keep changes in original document’ - which is ON by default. (The checkbox was introduced in 10.8.2; previously, on 10.8.0 and 10.8.1, changes would invariably be saved to the original document.) * ON means: save unsaved changes to the original document, then save the (same) current state to the new document. * OFF means: leave original document in the last-committed state (when changes were last committed either by saving explicitly or by explicitly closing the window (?? at least that’s how it works in TextEdit)), then save the current state - which includes the changes - to the new document.
The state of the checkbox will be remembered across session; I PRESUME, it is remembered on a per-application basis, but I have no way of checking that due to lack of another native, document-based application.