It turns out that Bungie hadn't planned on the additional "Combat Evolved" portion of the original Halo title. In fact, the team hated the idea - it was something that the Microsoft marketing team forced on the company.
Bungie was set to release the game as simply "Halo" when the Microsoft Reps said that it couldn't compete with other war games on the market. Halo Designer Jamie Griesemer was one of the people caught in the middle of the argument, and he had a few words to say about it to Edge Magazine:
"Oh, man... the subtitle. At the time, Microsoft marketing thought Halo was not a good name for a video game brand. It wasn't descriptive like all the military games we were competing with. We told them Halo was the name."
However, it turns out simply saying no doesn't work in all scenarios (like if the church up the street sends door-to-door recruiters at you almost every week. Totally a random example, though). Microsoft realized that changing the name of the game might cause a mass riot at Bungie, and came up with another alternative -which was still in Microsoft's favor, of course.
"The compromise was they could add a subtitle. Everyone at Bungie hated it. But it turned out to be a very sticky label and has now entered the gaming lexicon to the point where articles that have nothing to do with Halo get titles like 'Skateboarding Evolved'. So I guess in hindsight it was a good compromise."
Bungie would later go on to leave Microsoft entirely, and is releasing their final Microsoft-related project soon. Halo: Reach (they agreed on that subtitle, right?) will be out September 14th!