Fortunately "Save As" is coming back in Mountain Lion
, albiet only as a Keyboard shortcut.
My regular use of this (for the past 20+ years!) has been to create a new document with a different name (often after having made some edits), leaving the original in the state in which I opened it (or last deliberately saved it), thus avoiding having to go to the finder and do it there:
Snow Leopard Workflow
1. Choose Save As
2. Name file and hit Return
1. Choose Duplicate
2. Click "Duplicate and Revert" (to get the original back to original state at opening)
3. Choose "Save" on the Duplicate
4. Name File and hit Return
5. Click back to original document
6. Close Original Document
At least there is a "Duplicate and Revert" Option - saves one step reverting the original to it state at opening.
With all these steps in Lion it is certainly easiest to find the file first in the finder, duplicate it there and name the copy - but you need to know ahead of time that you are wanting to do this - my experience is that I'm often in the file before I decide that I want to fork the version with a new name.