I'm so bad at this in multi
It's actually sort of refreshing
A few questions for the vets
Melee attacks: How do they work? I'm assuming different weapons have different damage, is this correct?
Yes, heavier weapons typically deal greater damage. Grenade launcher melee from the earlier games were nearly guaranteed one-hit kills, for instance... but sloooow.
Do different weapons also have different ranges?
Yes I've been playing a lot of COD4, and that has a very noticable lunge when you knife someone, I repeatedly whiffed melee strikes, despite rushing in at close range while swinging (my opponents do not seem to have a similar problem

)
You get a feel for it, and there are certain weapons that will cause you to lunge forward as much as ten feet or more. COD4 may have learned that trick from Halo 2 actually.
Does hit location matter for melee damage? Rear hits deal more?
Yep, there's a considerable damage bonus for hitting someone on the back of the neck.
Am I missing something, or is dual wielding just pure rape at close/close-mid range? At mid and longer, I was vulnerable to either grenades or longer range weapons, but up close, decent dual wield combinations would tear me or my target apart.
Can't say for certain on Halo 3 but yes, in Halo 2 dual-wield would overwhelm everything else at medium range and closer. That's one of the imbalances they were supposed to have fixed this time around.