In this week's Bungie Weekly Update, Tyson "Ferrex" Green gives an exhaustive explanation of how Halo 2's melee system worked, how Bungie addressed its flaws in changing the design for Halo 3, how that design change led to complaints from the the playerbase, and how the Halo 3 Auto Update, due out later this month, will address those concerns.

Bottom line would seem to mean more simultaneous kills, and fewer situations in which the outcome of a near-simultaneous melee contest seems random:

For those of you uninterested in the precise details, the upshot is this: if you close for a melee attack and are at a clear advantage (or disadvantage), the outcome will be clear. If the outcome is unclear, too close to call, you will likely trade kills with your victim. But you should no longer watch your opponent saunter away for no clear reason (and if you do, check the film—it tells all.)

The update is chock full of other technical discussions of things like the Commando shoulders bug, cheating, host migration, and the like.
In particular I found the spread of hit points for the Halo player-- 45 for the body and 70 for the shield-- to be interesting.

I had always thought that since the elimination of the health bar and health packs after Halo 1 that far more hit points were on the shield than the body, but the difference here is less than 100%. And they recharge, too, which I was never sure of.

There's also a Mister Chief masterpiece made to benefit charity. Even if you stopped reading the BWU because Halo 3 came out, you'll want to read this one.