Zoomie, I always wear my regular trainers for anything longer than 10k.
At longer distances a light shoe isn't going to save me (on any other non-elite) much time and I would rather have the cushioning over that distance. I normally only wear lighter shoes for 5 and 10k. I have a HM on April 5 and I still haven't decided which shoes to wear because I am tempted to wear my new DS Trainers.
I will say this...if you decide to wear something in a race that's different from your regular or long run shoes, make sure you do at least one long run in them before the race. I know two people that bought fancy flats before a marathon and didn't run anything longer than about 12 miles in them. Both got nasty blisters around mile 16 or so. One DNF'd and one barely made it with a horrible time.

I don't consider DS Trainers a true racing flat though. They are not as beefy as a 2130 or as supportive and they are lighter, but have way more to them than a simple "flat".