Ok, since it seems not everyone on this board is familiar with this rumor I will give you a little of the reasoning behind it.
It is mentioned/implied several times that you can "join" the Sith, most people at first assumed this was like what you did in KoToR and become basically a rogue Dark Jedi. It has also been mentioned that they will have some sort of master/apprentice system in this game. Also there is the 3 Sith Lords and the 3 new prestige classes.
Ok it is a given that at some point you will probably have an apprentice (this is an easy way to come up with getting another Jedi in your party. They can't all be just commanded by the Jedi order to do it for no reason, or be bored old men.) But since we do not start the game as a prestige class, at some point we are obviously going to have to have a Master teach US what we need to know. I'm sure we will somehow "find" some semblance of a Jedi Council with at least 3 masters, coincidentally one of each prestige class. If we were a lightsider we could learn from them, become a master (after doing the few obligatory quests) and get our apprentice. But why would the Sith have different names for the same class unless they were actually different? That leads to the thought, that to become a Sith prestige class you will have to find a sith Lord of that particular class, coincidentally we already know that there will be 3 Sith Lords in this game... after doing the obligatory quests (and probably killing "your master") You will become a Sith Prestige class and get your own apprentice.
And as to which class each Sith Lord is, as I stated that was all just conjecture. I personally think that Sion will be the Sith Lord, all that Force to hold his entire body together wouldn't be an easy thing to accomplish. But most people seem to see him as a lightsaber weilding badass.
I also personally think that Mask will be a maruder, since HE seems to me to be the lightsaber weilding badass type, but most people associate the mask with an assasin who works from the shadows and wants to remain hidden.
So feel free to speculate (or continue speculating) since it could be me who is right, or it could be you, or it could even be everyone else. =)