Just announced, Call of Duty 6 will be known as Call of Duty: Modern Warfare 2 and will arrive some time in 2009.
Activision Blizzard made this announcement along with others as part of their "Massive Inc. upfront". An upfront is an event where publishers show off their upcoming titles to advertisers and the media.
Details about Modern Warfare 2 are not yet known, but expect more news in the upcoming weeks and months. It is anticipated that Modern Warfare 2 will be handled by Infinity Ward.