No, you can't become king because you don't have noble blood.
That is the dumbest thing I've ever heard!
Do you think Tiber Septim started his rise to power with a silver spoon up his #$$? All dynasties start somewhere. Better the poor hungry warlord than the already established noble be king because the former earned their kingdom through their own toil while latter did little to earn it. Read up on Titus Mede if you're unsure.
Okay let's talk about "noble blood". The forthcoming hero of Skyrim possesses blood worth far more than a hundred nobles put together. Why? Because the hero is a Dragonborn--descended from that same sacred and powerful blood line / legacy that Tiber Septim and his descendants were apart of.
Now with that aside. I think it would be great if the Dragonborn had the *option* of becoming king with all the wars, upheavals, and intrigue it will bring...if they choose. I think it would be a great idea if Bethesda weren't so concerned about their precious continuity.
I don't want to compare Skyrim to Fable 2 and 3. Fable 2 was a good game but the kingly aspects were ill defined. Fable 3 had the monarch system in place but it was not as good as Fable 2. I'm hoping that Skyrim will be a great game and allow you to be whatever you want; pauper, prince, or king. Either now or in some future DLC or expansion.