I would guess this has been posted, blogged or forumed about before and since I'm relatively new here I apologize to the TA vets if this is a repeat post.
However I still can't get over the simplistic beauty of the Lenski experiments. It is perhaps the best contemporary evidence and ammunition for atheists when it comes to arguing evolution.
A brief summary of the experiment:
Since 1998 Richard Lenski and his team at his Michigan State lab, have been watching e.coli evolve. Yes, evolve. Right before their eyes. This evolution happens because e.coli go through living and dying in an extremely fast way. Recently the experiment crossed the 50,000th generation barrier! The results have been dramatic. The e.coli have evolved and in doing so have acutally adapted to their environment.
So when discussing proof of evolution with a believer I would encourage everyone to read up on this experiment. Two great resources about the experiment are found 1) in Richard Dawkin's most recent book, The Greatest Show on Earth: The Evidence for Evolution and 2) on Richard Lenski's experiment's website http://myxo.css.msu.edu/
where you can see a live ticker of new generations and read the latest news on the experiment. Really incredible stuff.