I guess it will depend on two things:
1) What cd player you buy
2) What file type you listen to with your iPod
If one is listening predominantly to 192 kbs mp3s on the iPod, then yes, moving to a cd player is going to make a difference. IF one is listening to lossless or wave, then it will be less noticeable and in some ways, the sound could be better because of reduced jitter.
However! a really solid cd player (like the Music Hall M25 or whatever they call it, basically the Shanling S100) which is now selling for $319 is an incredible deal and often takes down cd players costing 1k+. Add in some mods later on for an extra 3-400 bucks and you have a player that easily competes with 1k+ players of all types and gets into the 2k+ level, maybe beyond? I have not heard one, but from the claims...
If one has the money, and the CD's, then I would not think twice about it. I find that the front-end makes the biggest difference once the headphones are chosen. I think one would notice a good level of improvement with SR 225's, perhaps not as much with lower level headphones like the KSC 35's (which are still my favourite budget phone)