As far as I know this is not something you can accomplish on the Mac at the OS level, but Soxy, from Rorohiko.com, can open existing ID files in the correct version when when you double-click them in the finder. If you don't launch 5.5 to start a file, that should probably be sufficient for ID if you also use Edit With instead of Edit Original to handle editing links. I have no idea if there is anything similar for Photoshop or Illustrator, but you can get out of the habit of double-clicking and either open the app first and use the dialog, or drop the file on the icon for the version you want to use.
Thx Peter. I will definitely look into Soxy. Re: dra & drop into the dock, I can certainly do that and have been doing that, but for workflow purposes and out of habit I would prefer just double-clicking jobs as oppsoed to dragging them. Anyway, thanks again!
Right -- the other option is to control-click and use the "Edit With" contextual menu that shows up. I find that slightly more convenient than dragging to the dock, but it depends a little on my mood :-)
Just a little tidbit: our Soxy also supports Photoshop and Illustrator (and a number of other file formats). Same deal as with InDesign: direct the file to the desired app when double-clicking the file icon.