2.7 unique users on Halo 3 that connected to Xbox Live.
Microsoft stated in a previous press release that 40% of consoles are not connected to Live even through a Silver account.
2.7m = 60% of Halo 3 owners.
4.5m = 100% of Halo 3 owners? Nah, probably not, but there could possible be between 3.5-4 million sales in the first week. Very impressive.