Good Hunter vs Good Warlock.... depends on spec, if you're not BM Hunter you are probably going to lose, fear spamming and DoT you to death is a warlocks way to beat anything one on one and if you're not immune to fear you're gonna get caught in it.
Beast Within is the only anti-CC a hunter gets, so the best way to beat a warlock.... send pet in, pop Beast Within, hit intimidate and aimed shot him, arcane, multi, steady.... basically dump as much mana into your shots in the 3 seconds he's out from intimidate and hope you get a crit or two... after that it's just keeping the damage higher than his drain life spell, or using Viper Sting to drain his mana and make it so that he has none and maybe he's stupid enough to life tap to get more while you keep Viper Sting up and he kills himself making your life easier.
Thats one on one, in any BG or Arena, warlocks are always the first to go down (focus fired) when I see them, otherwise their DoT's coupled with Direct Damage can take everyone down to half and one person out completely if hes's not taken care of quickly, granted healers can fix this fast but the fact that they can cause a healer to use 1/3 or more of their mana just from spending 2-5 seconds casting makes them very deadly in PvP.
Just for the record though, in my experience good Warlocks are almost as hard to find as good (I mean really good) Hunters, so some fights are going to be hard as hell other times you won't even break a sweat.