Here is an interesting comparison of Cortana over the course of the 4 halo games:
PTesh adds his dulcet prepubescent tones over the top of one of the new trailers for weapons...
Try upgrading your browser to Google chrome (you wont regret it). There is probably an IE...
Level: Halo 3 Trailer
In the Halo 3 Announcement Trailer, a 'Grunt Crossing' sign can be seen. This was also shown in the "E3: Making of the Documentary" video released by Bungie. Bungie's official response to this was: "The New Mombassa City Council had an incredibly efficient public works department until and even during the tragic and devastating Covenant invasion". This has got to be one of the cooler Halo 3 Easter Eggs out there.