Why not?
That's a big mystery. We thought it would be ok, because it worked the last time we played it at Den's house (his Elite 360 and my Premium) using a platinum hits version of CoD3 and an original copy.
On Friday, we used my Premium 360 and our friend Jeff's Elite 360 and it didn't work. We tried using the original copies in both consoles and it still didn't work (where it worked previously when we hooked up to 2 Premiums, mine and our friend Ray's).
The only conclusion we came up with was that Jeff and I haven't played against each other online yet, so our consoles haven't "talked" to each other to know that the game should work in multiplayer on each console.
I know. It doesn't make sense. System link should work offline regardless of console or version of the game. However, it seems to be hit or miss at this time.
Strange.