(I made this into a blog entry since I got positive feedback immediately when I posted it in the forums.)
As was mentioned many times before, the evolution of one species into two can be observed in the lab. I believe they usually use fruitflies.
The theory of evolution says that what works in the lab explains what happened in the real world as well. There is no reason to assume that fruitflies (or any animal) stop reproducing just before two populations stop being compatible enough to interbreed (definition of species).
There can never be "proof" for a theory because that's not how the real world works. Empirically, every single change we observe could be explained by evolution, but perhaps there was one event where a new species was created in a different way. We cannot disprove that. And doing so is not what science is about.
Now, Creationism and ID claim that some creator created all the species. That claim may or may not be as valid as the claim of evolution.
But there is no evidence for a creator in the real world and there have been no lab experiments that demonstrate how "creation" or "intelligent design" work.
So until ID proponents can create two different species in a lab, it's not a scientific theory to state that ID explains how it happened in the real world.
Let's use a different example, with a real world and a lab. We are trying to find an explanation (a "theory", if you will) for the facts we observe.
Our fact: In our real world in this thought experiment we observe that the smurfs are born in the basement of the house (there is only this one house in our thought experiment real world) but that some of them live in the first and second floors of the building.
Two explanations for how they get from the basement to floors 1 and 2 come to mind:
1. They use the stairs.
2. They are moved from the basement to the first and second floors by a creator or some such person.
Our lab: We have a much smaller building with mini-smurfs (mini-smurfs are very very small smurfs) that we observe. It has a basement and an upper floor and stairs.
What we observe in the lab: Mini-smurfs use the stairs to get from the basement to the first floor without our doing.
What we don't observe: A creator takes mini-smurfs and moves them around without our doing.
Our theory: In the real world smurfs ALSO use the stairs to get from the basement to other floors.
We cannot prove it. Perhaps they do. It seems likely that they do. Perhaps they use the stairs so rarely that we cannot observe it in our short lifetimes.
The point is that the _theory_ explains HOW it might work and we have evidence (the stairs) and can demonstrate in the lab that the method DOES work.
That's the difference between a scientific theory and a fairy tale.
Plus we don't have to have an explanation for why the creator exists or moves smurfs around.
There is NO evidence at all that G-d is running around the world stopping fruitflies from reproducing just before two fruitfly populations become two species of fruitflies.