I've written about intelligent design several times before, and I will continue to write about it. As long as people consider intelligent design to be a valid scientific theory, I'll say that it isn't.

Because it's not.

Intelligent design forwards no hypothesis of its own. It merely calls into question natural selection, an existing hypothesis of how the evolutionary mechanism works. (Again, it is not evolution that is in question, here; it is the mechanism.)

Well, that's not entirely true. There are several theories advanced by intelligent designers, but they are not empirical. They use logic and reason to come to some sort of non-empirical, logic-based conclusion about the nature of evolution. But the point of intelligent design is not to come up with an alternative theory to natural selection; it's to publicly de-bunk natural selection. Intelligent design is very much a political front for conservative Christians who object to natural selection. Intelligent design never answers the obvious question "Who is the intelligent designer?" The de facto answer is that it is the Christian God, and everyone nudges and winks, since they all know this to be the case.