I also have noticed that Halo 3 has two SKUs consisently in the top 15 at Amazon and the Legendary is always in the top fifty for the past few weeks...
Halo 3 is even ahead of Metroid Prime 3 right now which is even more stunning as I thought MP3 would easily beat it for at least this week.
I don't know where they are at in total pre-orders but the marketing monster seems to be getting everyone hyped really well.