I figured out a way to do this. It isn't perfect. It's limited to being useful against solid colour backgrounds. If you're using a gradient background or a complex image this won't work.
Add a mask in the form of a rectangle over top of each bullet point. You don't need to be super precise. Just get the rectangle to cover all of the bullet point text. Do one rectangle for each bullet you want to reveal. I used an obvious colour in the screen show below so you could see what I mean.
Open the Inspector window and select actions. For each rectangle set the action to dissolve and set the triggers to be however you'd like (on each key press, one after the other, etc.).
Now change the rectangle fill colour to match your background and set the opacity level of each rectangle to be something dim. 80% or so should do it.
Now when you play the slide, each click will cause a rectangle to disappear revealing the text below it at its full brightness.
Here's a Keynote presentation that illustrates the concept for you to play with. And here it is in action: