According to Bioware's website for the game, Star Wars: The Old Republic will have two distinct "jedi" flavors; Jedi and Sith. In addition there will be "A variety of other classic star wars roles" whatever that means. Until more information is released, any discussion about classes is pure speculation. Of course that doesn't make for a very interesting blog post, now does it? So speculate we shall...
While nothing should be assumed about Bioware's latest project, lest hopes rise to be crushed under the weight of a game that already has high expectations after Sony's mishandling of SWG, we can make some educated guesses. The problem with guessing is that with virtually no information out about the game, it is very hard to find jumping-off places. At any rate, away we go...
Scenario 1: Single Jedi classes for each empire with a "variety of other classic star wars roles" This is by far the easiest and most cut and dried form of the game. Basically Jedi/Sith acts like any other class, except you get a glowy sword. The game will feature, most likely, a variety of classes along the roles of tanking, healing, cc, and dps with Jedi/Sith filling either a tanking role or some form of dps/cc. If there is any customization with the Jedi class it is likely to only be in talent selection (if using three talent trees, the consular/guardian/sentinel trees would be the obvious choices). The upside: jedi is playable and customizable out of the box. The downside? It sucks running around with a billion other jedi and trying to feel special. Also, everyone and their brother will be a jedi, leaving the other classes largely useless.
Scenario 1a: Multiple Jedi classes for each empire with a "variety of other classic star wars roles" included. Same as above except you make each jedi/sith type an individual class. The upside: More customization and making Jedi feel different slightly. The downside: Even more reason for everyone to play jedi. I should note the obvious problem with both of these systems is that Jedi will have to be in balance with all the other classes, which de-powers the jedi and will probably make them less fun to play.
Scenario 2: Jedi as an unlockable class. No matter the form this leads to players being forced to play "lesser" roles to become jedi. Basically you either play until you are max level and are rewarded with a jedi, complete quests to unlock jedi powers and then go through training, or some other general mish mash of that sort of thing. You either become a jedi on that character or can now create jedi on that account. In either case jedi becomes playable for you, but only after you reach X requirement. The upside: Fewer Jedi at release, though numbers swell rapidly once people realize how you unlock it. The downside: people will complain about how hard it is to unlock jedi, the longer the game is out the more jedi there are, balancing a class that should be more powerful (because it is a reward) but still able to be defeated by other classes.
Scenario 3: Jedi is not in the game. Easy way around the problem is to remove Jedi and keep classes along the standard tank/healer/cc/dps line. Makes for a game that is WOW in a different set of models. The upside: it's ridiculously easy to create a game without the iconic figures of star wars in it, and you don't have to make all those fancy lightsaber fight moves. Also, it is easier to balance and dodges the whole jedi bullet. The downside: Would you buy it if there were no jedi? Didn't think so.
Scenario 4: Jedi is the only playable class. Another way around the problem. Everyone is a jedi. Jedi then just differ through powers or talents. The upside: Millions of fanbois will buy it, no need to balance the game, because there's only one class. The downside: Again, would you buy a MMORPG consisting entirely of Jedi? Also, everyone will be a jedi. Did I mention that Jedi will be common as fleas? Oh, and Bioware already said they would have a "variety of other classic star wars roles."
In my opinion, Bioware will most likely select option 1a, as it fits their KOTOR model of dealing with Jedi. Jedi will probably be slightly better in a PVP aspect than other classes but will have a harder time with PVE content. This provides the ZOMG awesome aspect of Jedi with the balance that games need. I will be watching with great interest.