They created a repeatable experiment that shows evolution taking place. As certain of evolution's truth as most scientists have been - particularly in the absence of any other serious theory - the inability to repeatably show it in a lab has been a weakness in the theory of evolution since it was first advanced. It's something that tends to plague observational sciences in general - astronomy comes to mind, since we're trying to form theories about the universe based on the scant bits of light that make it to our planet - and though we can get our hands on these organisms, they're generally sufficiently complex that it's hard to isolate factors for experimentation. But now we can, and I look forward to further details - and further experiments based on changing variables - regarding how exactly the bacteria evolved.
One more argument against evolution, struck down in the face of experimental results. It's a good day for science.