This is a guide on a PvP DK spec I like to use in Lordaeron. I can't warrant that it works in other WotLK realms due to possible class bugs.
Generally speaking, Unholy, being quite utilious, is the most played DK spec in PvP. Blood and Frost are also an option until 2k in arena, world PvP or BGs but most players still favor Unholy.
I, however, don't. I've always like the whole idea of invincible Blood in PvP. Blood DKs are great in melee but they have a lot of trouble with casters. My way of solving the problem is likely familiar to most of you - a deep Blood spec with permament pet. Many players dub it a hybrid spec but it's purely Blood with pet if you ask me.
Let's take a look on the talents. First, we need 25 points in Unholy in order to get Master of Ghouls which gives us the permanent pet. I recommend some standard Unholy PvP talents: Vicious Strikes, Virulence, Epidemic, Morbidity, Unholy Command, Ravenous Dead, Outbreak, On a Pale Horse, Night of the Dead, Unholy Blight and Dirge. Max out all these and u should have the required 25 points for Master of Ghouls. A few words on the talents - this is probably the best way to reach 25 points in Unholy. Otherwise you will have to choose DPS talents such as Necrosis. You won't be spending enough time on your target to make auto-attacks worthy at all. Unholy Blight is essential for PvP and shouldn't be overlooked!
Frost is best skipped and we move on the real thing - Blood. I will focus more on these talents.
1st row: Butchery, Subversion - Subversion is crucial for special attacks and Butchery helps pull off more Death Coils. Blade Barrier is a tanking talent.
2nd row: Bladed Armor, 2H Weapon Spec - both talents boost your damage. Scent of Blood is not efficient in PvP.
3rd row: all - first we get a neat heal and more crit. And then we can unleash up to 8x Heart Strikes in a row thanks to Death Rune Mastery. This will of course hardly happen.
4th row: nothing - Improved Rune Tap is not worth the 3 points and the other 2 are tanking talents. If you still need more survivability, you can swap Death Rune Mastery for Improved Rune Presence.
5th row: all - overall stat increase + Mark of Blood a.k.a. The DK Bubble. Make sure to use it against Rogues and Ferals.
6th row: all - damage increase.
7th row: Hysteria, Improved Blood Presence - Hysteria is our burst and we're going to sit in Blood Presence ALL THE TIME! Bloodworms is for tanks.
8th row: all - damage and healing increase + free Death Coils. Only put 2 points in Sudden Death.
9th row: the last 4 points go to Hear Strike and Might of Mograine.
Glyph of Death Strike, Glyph of Heart Strike, Glyph of Death Coil and Glyph of the Ghoul are good options. Choose 3/4.
Stat Priority: Hit > Exp > Spell Pene > Resilience > Armor Penetration > Strength = Crit = Attack Power
164 Hit Rating and 26 Exp should be enough. Don't go for more than 195 Spell Pene.
An important enchant is the Tuskarr's Vitality for boots. For the rest of your enchants and gems - follow the stat priority.
Professions are not game-changers, but Engineering is cool and Tailoring has a Parachute (make an extra Parachute Cloak for Arathi Basin's Lumber Mill).
If you need more details or you have questions or suggestions, the forum is yours!