I'll agree with J DUB on this one. I spent a month doing cardio first thing in the morning (...no I don't mean in one go!), I got excellent results very quickly, but I put this down to a number of reasons.
I have also read, training first thing in the AM on an empty stomach speeds your metabolism up and allows your body to burn food more efficiently for the next few hours, and I can honestly say that I did feel, er, thinner and less bloaty for the rest of the day while training like this.
I think I only felt this way, as by the time I had my first meal of the day, I was already starving, so after stuffing back breakfast, and being hungry again within an hour, I guess you believe that you have 'burned off' that meal, and your body needs more calories again, so is gonna have to gnaw away at fat reserves until your next meal.
I also feel thinner and less bloaty after doing cardio (I'm built for weight lifting, not running), so after running I have this pyshcological thing, where I do believe I have miraculously shed all my bodyfat, so therefore think the bullshit myth is true about your metabolism running flat out all morning.
The truth of it is that my results are due to me actually getting off my ass and doing cardio rather than slinging the weights around. If I go to the gym in the evening to do cardio, and I see the guys in there lifting big weights, I'll say fuck the cardio, and go do weights with them. There's no disruptions early in the morning, and if I drag my ass to the gym that early to do cardio, then I'll train my nuts off as there's no way i'm missing out on sleep for a wasted session.
In case anyone wondered what cardio I did that I found effective, I did 4 - 5 session (AM) of 1 hr on the treadmill. Not wanting to lose muscle, I did stay in my 'fat burning zone' (no more than 140 absolute max(except for the interval training bit), BPM), 30 mins speed walking followed by 15 mins interval training, 1 min on (at warp speed), 1 min off (can vary this), followed by 15 mins of walking on a steep incline.
Like I said, my body responded to this probably because of the fact I was actually do cardio and sticking with it for a change.
Lame answer I know, but I reckon it's a balance of people having a structured cardio routine - eg, actually training in the morning and placebo effect.