Here is my prediction (which means it's a fact)
Halo: Reach will offer gameplay elements that satisy both Halo/COD fans alike. So the amount of unique users per week will sit around 1,900,000 for the first three months and Halo 3 will be a lot less than it is now. Gradually however, the Hardcore halo fans will come back to Halo 3 and takes turns between the two because they miss the classic Halo gameplay, and will eventually balance out with the unique user average being 1,200,000 for Halo: Reach, and 400,000 for Halo 3.
Yes, this is fact. You'll see.