The ultimate strategic goal for Bin Laden and his followers is a world of Muslims living under Sharia law. It is a long term goal to be sure. But getting the US out of Saudia Arabia and all the rest is just short term goals. Ultimately they want to see the US converted to Islam, and living under Sharia law. The language is somewhat stylized, in accordance with their religious nature, but fairly clear if you will see. The demand for conversion is pretty straightforward, and the call for Sharia only slightly diffused but you can see it - and it follows naturally from the conversion demand.
Beware the person who uses weasel words like "ultimately", "fairly clear" and "follows naturally". It means he's trying to pull a fast one. Beware also a poster who selectively cuts and pastes passages out of context to push his own agenda. I strongly recommend that anyone who has an interest in the matter to go read the full, unedited letter rather than rely on someone who is, as others have already pointed out, likely to be a paid government shill.
To show you the dangers of citing out of context, I point you to the following passage of the same letter
We call you to be a people of manners, principles, honour, and purity; to reject the immoral acts of fornication, homosexuality, intoxicants, gambling's, and trading with interest.
Doesn't that sound very much like what some of the bible belt conservatives in the US are pushing for? Can I then exclude everything else in the letter and based solely on that one passage, argue that the Osama group's ultimate goal is to turn America into a land of "manners, principles, honour, and purity"?
Because this manner of argument is exactly what the previous poster has done.