From what I have gathered Halo Reach's ranking system will be slightly different from the previous halos and will look more like a time vested system a la Modern Warfare-ish. Here's some info from the recent bungie update:

Rank - Rank as you knew it in Halo 3, is dead. The concept of Rank has been revamped and divorced entirely from Trueskill (more on that in a few paragraphs) and the concept of EXP (at both the Global and Playlist levels) has also been discarded. 

In Halo: Reach, Rank is a global, cross-gameplay-mode-encompassing representation of the time you've spent in the game. So whether you're a Campaign-only guy, or a Matchmaking-only girl, or you swing both ways, you're going to be seeing that Rank advance as you keep playing.

Rank will increase over time as players earn more Credits. Credits are the lifeblood of the Player Investment system and they can be earned in all of Reach's gameplay modes, through a number of systems. 

Finishing games and Winning Games in Matchmaking - For the purposes of the upcoming Public Beta, this is going to be a significant source of earning Credits. There are very clear constants in the ways that Credits are earned in Reach across game modes and at the heart of those constants is Time. In the long run, the majority of Credits will be earned from this bucket.
Commendations - Commendations are persistent, aggregating awards that will advance through gameplay. When players are progressing Commendations, they're going to be earning Credits - and when Commendations reach milestones, players will earn a bunch of additional Credits. Most importantly, Commendations serve as virtual scouting reports with information on what type of a player someone is, what weapons they use and what tricks they bring to the table. Commendations in matchmaking won't aggregate specific actions - like say, a Sniper Headshot - but instead, a Headshot. The Beta will have a small subset of Commendations at very limited levels of completion.
Challenges also contribute to Credit earn (among other things), but there's a question coming up about those later, so I'll save it.
Achievements - Spoilers! There will be Achievements in Reach and completing an Achievement will give you some small amount of Credits.
And more...
For more information on the new ranking system and changes check out the full update here.