I just have some point counter-points for the examples of Luffy's possible use of Haki early on in the series. I'll mainly be sticking with the examples already provided, i.e. the Crocodile and Arlong battles.
Now there have been some good arguments thus far about whether or not it was Haki, and I'm just adding in some points for consideration:
For Haki
Crocodile has trained his logia abilities to go intangible on reflex
The sand burst on impact, and had barely enough time to slice Luffy's flesh
Luffy is rubber, and stretches consciously or not, plus Arlong is certainly strong enough to flip a town, I don't care how much stronger Luffy was due to emotions, Arlong should have been able to at least budge the sword away from the rubberman
Arlong also was having a bloodlust effect (if my memory is correct), so he would have had a boost in strength as well
From what I've read of Haki it's a natural extension of abilities that humans have basically forgotten over the years, which can be unlocked due to stress, shock, or very intense training (and considering what the OP definition for 'intense' is, I'd say that the training basically covers both [seriously we've gotten a glimpse of what Garp's regime is like, likewise with Zoro and Zeff, whose to say Roger and by extension Rayleigh aren't similar])
Against Haki
Crocodile was weakened and in a fit of rage during that battle, and the mind has overcome reflex on occasion
Luffy was covered in sweat, blood, liquid poison, and water at the time so it is possible that this is what helped defeat Crocodile (however it doesn't explain the bursting sand, if anything it'd make it more solid)
Arlong was previously dealt massive blows by Luffy, and had to swing that huge (and possibly extremely heavy) sword up all the way to the top of the tower
The sword in question also seems to be a last resort weapon, or else Arlong would have used it long before he did
Luffy is pretty simple, either due to his DF ability, or something from his family that skips a generation, and is highly emotional. Speaking from experience as an emotional person, adrenaline does temporarily push a person beyond their own limits, sometimes at the cost of torn ligaments. Since Luffy is made of rubber he go beyond the normal risks, which is probably how he came up with the Gears abilities.
Also after having pondering this dilemma for sometime (how does a rubber person keep from stretching every which way when walking, attacking, moving, etc), I believe that a rubber person has to continuously, whether conscious or not, tighten their rubber muscles in order to make sure that don't accidentally stretch and ricochet their limbs.
There are my points, if any of you wish to debate them, go ahead