Anyone who equates intelligent design with creationism is misinformed. Intelligent design is creationism, pure and simple. Not the crude and primitive creationism of those who take every word in the Bible literally, and who therefore think that the Earth is just a few thousand years old and that there never has been an evolution of living beings. The modern version is more sophisticated, designed to pull the wool over the eyes of the layman. So they don't pretend that living beings are created instantaneously (like the old theory) - they accept the fact that evolution has occurred, but they claim that "some intelligent designer" has been steering evolution, in other words they propose "creation by steering evolution". In their official publications they are careful not to use the term "God", to avoid the accusation that they bring religion into science, but there is enough evidence that they do mean "God" when they're talking about the intelligent designer and that they do not for one moment think that it could have been some alien (which by their own argument would only lead to an infinite regression of intelligent designers).
Their argument boils down to: "we can't explain the complexity of this organ/part/system of a living being and therefore it must be the result of "intelligent design". This is the typical "god of the gaps" argument, and it is unscientific as it is unfalsifiable. Moreover, the examples they give of so-called "irreducibly complex systems" have been thoroughly debunked. So the ID theory is not a scientific theory, only a last attempt of the creationists to get a foot in the door to American education.
Just as it is perfectly acceptable in physics to ponder the origin of things, the big bang being the current favorite, it is equally legitimate for the biological sciences to do so. Unofortunately for biology the previously held theory that a bolt from the blue cooked non-life into a living cell is kaputI don't know what "previously held theory" you mean, perhaps you're referring to the Miller experiment in the fifties. If you think that biology hasn't advanced since then, you should do some reading. Even at the time nobody claimed that you could get a living cell by some lightning discharge in a certain atmosphere under the right conditions - the experiment only showed that you could get amino acids that way, and while that may have been suggestive, nobody claimed that this explained how you could get a living cell. There are several theories about how living systems may evolve from inanimate matter, but they're necessarily speculative, as it is unlikely that we can find hard evidence (such as the fossils of a later stage in the evolution). Perhaps we may once find similar stages on other planets. But we don't need any supernatural explanation, such as the unscientific ID "theory".