For PvE, Annihilation is a much better spec to use. Sadly there is no dual spec yet but I have heard it is likely in the future. So unfortunately you will have to make a choice to PvE or PvP. That's not to say you cannot run Carnage in PvE or Annihilation in PvP, its just that each is suited better for different things.
I will be posting up an Annihilation spec in the near future for PvE/Raiding.
Here is my current work in progress for a PvP build on my Marauder. Bioware has stated they are working on changing both Marauder and its counterpart Sentinel, So this build will likely evolve over time. But since there is no date for the changes, here we go. This build utilizes the Carnage tree. Why? well lets first look at the other 2.
Annihilation is a tree built around dealing damage over a longer duration. While Annihilation will shine in PvE, it is not suited for the fast pace nature of PvP; so Annihilation would be a poor choice.
Rage on the other hand is pretty decent for both PvP and PvE. It gives a large amount of crit bonus, it utilizes the Shii-Cho Form (a fairly balanced stance), gives a decent amount of armor pen, grants an extra short range charge, and a fairly good slow ability from range. Rage could be viable and I will look into it in the future.
But Carnage seems to outshine both of the other specs when it comes to PvP. I aim to utilize as much rage generation as a majority of your damage requires a constant supply of rage. Also, Carnage provides increased speed, burst damage, and consistent damage over time. so lets break it down.
Tier 1: Defensive Forms is a must for PvP. gaining the extra 10% movement for Ataru Form (which will be the stance you are likely to stay in) will help keep you in close range constantly. DW Mastery just serves to increase your DPS, and Cloak of Carnage guarantees extra rage when players are attacking you, adding this to Cloak of Pain just makes it an incredible PvP ability.
Tier 2: There isn't a ton of good for PvP in teir 2. Erupting Fury is more for PvE as you will not be generating a ton of extra fury with it in PvP (unless it is large open scale PvP) Narrowed Hatred is another nice PvE ability, but accuracy is generally not needed as much in PvP. So your choices are Defensive Roll and Enraged Charge. Both are helpful, but you only needed 3 points in T2, I prefer the guaranteed rage over less AoE damage taken(Depending on Rage generation end game, I may switch the points around).
Tier 3: Everything here just adds to your DPS. You finally get Ataru Form, which will be your permenant stance. Ataru Mastery ups the Damage from the extra swings with Ataru and Execute gives a small burst when ataru triggers. Take everything here.
Tier 4: Depending on what you feel necessary, you may leave out some points here, but here is my reasoning for taking what I did. Blood Frenzy is bonus rage every time Ataru triggers, which is needed. Towering Rage guarantees a Crit for your Force Scream, which is one of your hardest hitting abilities, so it is very worth it. Displacement will be highly needed in PvP to give you a root at range. Since you depend on melee combat, you need everything you can to keep people from running, A must take. Enraged Assault cuts off 3 seconds from Battering Assault, which is a large rage generator. You may find this unnecessary as 3 seconds is a pretty small cut for 2 talent points, which can be better used elseware.
Tier 5: Here is where Carnage really starts to shine. Unbound turns your stealth into a free CC break for most immobilizing abilities. Gore is a 100% armor pen bonus. This is a MUST HAVE as for 6 seconds you can turn a tank into squish. Rattling Voice lowers the cooldown on Force Scream AND lowers its rage cost by 2. As I stated before, this is one of your hardest hitting abilites thus it is very worth the points.
Tier 6: Again a very sexy teir, Overwhelm turns ravage into a root, giving you more time to guarantee damage to your opponent and making Ravage viable in PvP. Sever boosts Crit damage of 3 very important skills for your DPS: Force Scream, Ataru Form Triggers, and Massacre.
Tier 7: Being the top of the line, It would be kinda pointless to go this high in the tree and not take Massacre. It does good damage, force triggers Ataru (which in turn triggers a lot of passives for you) and gives a bonus 30% chance for Ataru to trigger for 6 seconds. Just an overall nice boost to your DPS.
NOW, what do we do with the remaining 10 points? Well we could invest more into Carnage, taking Enraged Assault if we feel it necessary, Erupting Fury if we think it will give us an edge, or we can go into other lines which is probably the better option. In Annihilation I would say Enraged Slash is the best skill to take. It refunds rage with 3 decently high priority abilities, which just serves to give you more ability to deal out dmg. Cloak of Annihilation is nice, but Retaliation is a reactionary ability so you may not trigger it a lot; also if you are the target in a fight then your Cloak of Pain will stay up for the entire duration. So it isn't incredibly useful. If you do decide to go up to tier 2, Seeping Wound will give you extra movement slow, and Hungering will give you a little bit of sustain. Personally I would take 3 points in Enrages Slash and that is it.
We now have 7 points to spend in Rage. Malice is 6% Crit chance... no reason not to have that. Stagger outshines Ravager from a PvP standpoint as it is an extra second of immobilize. Since Force Charge has a much lower cooldown then force choke, you will likely be using it much more and it allows you to get back into the fight. When you get to tier 2 there are 2 talents that stand out: Payback and Brutality. payback gives you 10% HP back when you use Unleash (your CC break) that is a good chunk and you will likely be using Unleash a lot in PvP. If you have the extra Points, Brutality isn't a bad choice with 15% crit chance bonus on Vicious Slash. I would still prefer Enraged Slash in Annihilation due to the rage return, but the choice is yours.