According to the studio, it is one of "less than 10" games to reach the 10 million mark since 2000. Although we don't know for sure what those games are, our best guesses are The Sims, The Sims 2, World of Warcraft, a half dozen Nintendo DS games and Wii Play. As of January 2008, Halo 3 was at 8.1 million units sold worldwide. (It should be noted that the Call of Duty 4 has an advantage here by merit of the PC and PS3 versions.)
As for Call of Duty 5, Activision has still not confirmed which franchise regular is developing the game, be it Treyarch or Infinity Ward. Rumors suggest the former, but given COD4's success, we really hope IW gets to latch onto the series and keep hold.
Update: We just got word from Activision that the 10 million figure does not take into account the Mac and DS versions of the game.