Although it's no surprise to find out that you will be allowed to reset/redo your skill choices in a MMO, Georg Zoeller, the Principal Lead Combat Designer for SW:TOR, chimed in recently to clarify to what extent you will be allowed to respecialize.
You will be able to respec your skill trees within an Advanced Class.
You may be able to spec to another Advanced Class, but that is something we haven't entirely made up our mind about as there are plenty of arguments that can be made - pro and con - and future class developments could influence this. We're definitely looking at the feedback from those in our Game Testing Program about this topic as well.
You will not be able to respec your class (e.g. go from Jedi Knight to Smuggler or Consular), as your entire class storyline would be compromised if we allowed that - the game just wouldn't make any sense anymore.
Thanks to Massively for the heads up.