@Beowulf89:
I'm of two minds in regards to Haki.
Basically, I don't think that Zoro will get any real advantage against Mihawk that has nothing to do with swordplay. So, if Mihawk has some Haki powers, Zoro will have at least some.
But I don't think Mihawk uses Haki. Because throughout the series, you see Zoro look down on people who call themselves swordsmen, and then rely on gimmicks. He won't let Usopp use Dials to upgrade his swords, for example. He didn't think of taking Funkfreed, despite losing one of his swords only seconds earlier, and went for generic, nameless blades instead. If Zoro found out part of Mihawk's strength was that he could know an opponent's action before he acted, he'd be disappointed. I think, by extension, just as Mihawk is simply "the best swordsman," Zoro will reach the top of the world, merely by being the best swordsman possible.
I both agree and disagree.
I think pure swordsmanship can get you to a certain point, and Mihawk is so good, he probably doesn't need to use anything else against most opponents (even against Zoro). However, start climbing the mountain of top-tier fighters and you're looking at individuals that can use techniques such as Rokushiki or powerful DFs like Logias. While I'm sure it was far from his strongest, one of his slashes was easily deflected by Diamond Jozu.
So I think that haki is not a substitute for, but rather a complement to, Zoro's sword techniques. He used only slashing techniques in the beginning of One Piece, but later he started using advanced techniques such as his 36/72/108-pound-cannon. Zoro also has a very heightened sense of proximity and movement, which is just as much an extension of his swordsmanship as it is an element of haki.
So in other words, I think haki doesn't function as a gimmick for Zoro, but rather an enhancement to his already formidable battle prowess and instinct (which Mihawk also has in spades–remember how effortlessly he predicted Luffy's Gear 2 movements).
Also it is clear from the manga that one requires haki to even touch Logias. It's the logical next step for all three of the "Monster Trio". Think--if Luffy didn't have a rubber body, his entire crew would have been effortlessly defeated by Enel alone. Even dispatching Zoro and Sanji was no problem for him, and I think Oda was showing us a preview to haki and its capabilities.