Halo 3 zoomed from its first day US sales of $170 million to a first week total of $300 million, according to GamesIndustry.biz. It was also heavily played on Xbox Live, garnering the top spot:

Microsoft stats reveal that more than 2.7 million people have played Halo 3 over Xbox Live in the past week, with the first day of release seeing a total of 3.6 million hours of online play, increasing to 40 million hours by the end of the first week.

Yowza. There are no unit sales figures yet, but if the average price of Halo 3 was, say, $90, to account for a distribution of Standard, Limited, and Legendary editions, that's 3.3 million copies. If there are fewer Legendary editions than Standard and Limited, then that number goes even higher.