Here are some facts (with citations) I have attempted to dig up about Osama bin Laden's motivations (his organization being the prime suspect un the recent attack). Rather than stating my opinions (opinions just seem to get people mad) I'll let folks draw their own conclusion.
According to this article, the main thing that angers bin Laden is the US troops stationed in Saudi Arabia at the request of the Saudi government. He believes that Islamic scripture forbids the presence of non-Muslims in the Arabian peninsula (citations for this in Islamic holy writings I have seen include Hadith Vol. 5:716 and Qur'an 4366, but these were found on sites that aim to discredit Islam so I cannot vouch for how these verses are interpreted by Muslims in general). This is one reason his organization tries to destabilize the Saudi government and attacks the US military presence there. Second on his list of grievances is US support for Israel, and third are US alliances with the more moderate Arab governments (such as Egypt, Kuwait, Saudi Arabia, etc).
In this rare interview, bin Laden expresses these views (all the references to liberating the Ka'aba are to the US presence in Saudi Arabia). He also seems upset at alleged support the US gave to the Serbs in slaughtering Bosnian Muslims; and later in the interview, for a plan he believes exists to split Saudi Arabia into three countries.
He also specifically includes Britain, along with Israel and the US, in the "crusader-Jewish alliance" that he opposes.
I strongly suggest reading the article and the interview to those interested in these issues.
(The reference to crusaders is particularly interesting since the Muslim conquest of Jerusalem in 638 was used by the Catholic Church to instigate the Crusades, which resulted in the slaughter of many Jews and Muslims. An amazingly sad and violent part of history.)