To further expand some thoughts:
1) We can never truly trust Islam. Their religious texts explicitly say that they do not have to act in good faith with non-believers. Any lie or deception is not only admirable, but expected.
2) Their culture was founded on spreading their religion by the word and the sword. Mohammed imposed Islam on Mecca and the Arabian Peninsula at the head of an army. This tradition continued under the Caliphs after him.
3) There is no reward for moderation in the Islamic culture. The way to power and wealth is extremism. Moderates are ignored or eliminated.
4) There is huge historical resentment in the Islamic culture. They remember that they were once the zenith of civilization. They cannot accept that their present backwardness is due to their theocracies. They do not yearn to achieve our standard of living, they would be perfectly willing to reduce ours instead. (the bucket of crabs phenomena)
5) Their religious leaders are not stupid. They know the dangers of Western liberalism poses to their position, power and influence.
6) Secularism is practically non-existent in the Islamic culture. The one Islamic country that pays lip service to secularism, Turkey, is still dominated by religious considerations.
7) We have seen in the UK, France and to a lesser extent the US, that even second and third generation immigrants will reject their host culture and embrace militant Islam.
8) Reason is not nearly as important in the Islamic civilization as our own. Our reliance on reason is an outgrowth of classical liberal thought. In the Islamic culture religious belief, raw emotion, honor and revenge are all more important than reason. Reasonable people do not become suicide bombers.
9) The Islamic culture is an Eastern one. They do not value individual life as we do. This is again due to the lack of classical liberal thought. They are still tied to the ideas of family, clan and tribe and are perfectly willing to subsume the interests of the individual to the larger whole. They see our concern for individual lives as a weakness to be exploited. This will make any eventual conflict between our civilizations even bloodier.
Update (2/7/06) :
10) Moderation and accommodation (both of which are necessary in order to co-exist with a competing ideology) require that you respect your opponents, and grant their motives and beliefs legitimacy. Islam does not and cannot do this. It runs directly counter to its doctrine and dogma. To the question of "Can't we all just get along?" the Islamic civilization replies, "Only if you convert."
Check out this link to an article that says much of what I have been saying from last August.