I have an '07 classic Duramax, which I only drive about 50 miles / week.

I noticed that when the temperature drops outside a odd thing occurs.

the lights flicker (outside and dash lights)
the voltmeter will seek between 14 and 16.5 volts, randomly

I took it to the dealer to have a look and they told me it was common due to the fact they are using new alternators which turn themselves off and on.

Has anyone else heard this explanation, or can you validate that I have nothing to worry about. I do trust this dealer, I just would like a 2nd opinion.

The only time my voltage is low is when the truck first starts up and the intake heater is on. It usually turns off about 2-3 miles down the road and then my voltage goes back up to normal.
I have never seen any of my lights flicker like you described.

Actually, I haven't seen this in a while, but my 2002 used to do it. Probably still does, but I don't drive it in winter these days. I don't recall the 2005 or 2007 doing this though.

The way I would describe it is the voltage drops down like it is immediately after startup when the alternator apparently is not being field energized. It pops right back up and may do this a couple of times. It basically occurs at idle/coast when braking in my experience.

mine does the same as idioteque's. my LED Recon lights hate it. also dont forget the grid heater pullin lots of power for the first few minutes after start up.

Mine does the same thing but only when it is cold. When I first start it is low, barely above 12v and then it bumps up to about 13.5 and bounces between that and 15 or 16v until it is fully topped off then it settles at a tick above the center line.
The stealer told me what they told you. The "new" style alternators only charge when needed to reduce load on the engine. I told them I thought that it was weird that it would "bounce" charge like that but they said it was normal.
IMHO, the regulator would get worn out bouncing around like that rather than regulating a slower more consistent charge and then shutting off when it is topped off.