These preceded the announcement of Halo and eventually proved to be a series of transmissions between a human computer and alien spaceship captains, covering events leading up to the opening scene of Halo.
gspawn: At the moment, all we have to really connect the Cortana letters to ILB is the fact that they prove Bungie is willing to go to great lengths- including cryptic emails that are authentic right down to an explanation of IP addressing- to keep their fans interested in the game. See the Microsoft / Beast connections, and it's easy to see that since Bungie now has a Microsoft member or two who worked on the Beast, they'd want to employ that skill for their fans.
In early 1999, Hamish Sinclair, the famed Story Guy for the Marathon's Story Page site, began receiving rather cryptic messages from an entity calling itself "Cortana". This was before Halo had even been revealed at MacWorld New York 1999, though rumors of a new Bungie game were flying about rampantly, as it were.
More analysis at http://rampancy.net/cortanaletters
In doing some Background reading about ARGs in general, I came across this website by Hamish Sinclair. In the entry for March 20, 1999, a letter from Nathan Bitner (a Bungie Employee) states that Bitner's anniversary with the company will be August 24 (1998). Which means that the date on the site will correspond exactly with the countdown from ILB. My own speculation os that this is just coincidence. -UserStrocce