I dunno if it was mentioned or not, but fuck if I'm gunna read all that(hell, I ain't even gunna re-type it- this is all copied and pasted from posts on other forums)
The Gravemind, as far as I can tell, is like an Overmind like creature- it controls the flood with a master-intelligence.
But for Cortana, she's Rampant.
Rampancy occurs because of 3 things; either she's gotten too old(unknown), gotten alot of outside stimuli(obviously, Master Cheif offered her this), or she's accessed a huge system, in which she has freedom to grow her systems with questions(Like Halo, Pillar of Autumn, High Charity, Delta Halo, ect)
There are 3 known levels of Rampancy(according to Wikipedia)- Melancholia, or feeling like a slave, and theres nothing anyone can do to help it. Apparently, this stage can last for many years. Cortana was displaying those attributes. In Halo 2, she isn't as chatty, isn't really all there....
The second level is Anger. This can easily be seen in Halo 1(so the first level of Rampancy could have been happening even before we met her) when she fights with the Index- her anger is extremely clear.
The last is Jealously, which we may not have seen yet....
Equally, the new trailer shows Cortana degrading and breaking up- something the rampant AI's in Marathon did- and Halo 3 concept art has this;

http://xbox360media.ign.com/xbox360/image/article/707/707165/halo-3-20060510005847408.jpg" width="700">
According to Wikipedia, following the Rampant ideals of AI's in Marathon, they said;
Given that rampant AIs typically concoct plans which are surprisingly beneficial to those who think the plan will doom them, Cortana's conversation with the Gravemind may be a plot similar to this.
One huge sign of Cortana's Rampancy is this- she never blew up High Charity to stop the Halo from firing. You noticed that? She was supposed to save the Universe from complete Annilation, and she let them go ahead and do it. Why?