i have been interested in morning workouts vs afternoon workouts for a while. while i prefer afternoon workouts so that i can sleep in, i think that there is definitely a positive aspect of working out in the morning. since your glycogen stores are somewhat depleted, it makes your body adapt to this. on the other hand, if you are eating all day, you have all of your energy for the afternoon. i know that chrissie wellington, who just won the past two ironmans, is a big fan of glycogen-depleted training. in addition, if you train in the morning, you have all day to walk around and refuel. if it is an evening workout, you basically eat dinner and go to bed.
basically, im not sure. but it would be interesting to do a study on the differences.