Laptops are indeed powered off of DC, which means that there will be a constant voltage measured across the positive and negative pins of the MagSafe plug.

However, the voltage measured from either pin to ground might very well fluctuate. Without the grounding pin on the power supply, the DC side of the supply may 'float' with respect to earth ground. If you plot the voltages measured from each pin to ground, it probably looks like a pair of sine waves, one shifted 20 volts above the other.

Right, so there's no relative phase change between negative and positive terminals but there is a phase change between the terminals and ground (which can be interpreted as a frequency relative to ground)? And since I'm grounded I experience that frequency? Interesting. Is there a relation between the "float frequency" and the AC freqency? Is it the same because of how commutators work?

I suspect that the float frequency is generally equal to the AC frequency, but I suppose it may be possible for it to be some subharmonic of the switching frequency, assuming you've got a switching power supply.

The socket isn't grounded the laptop on your desk and you are, so you get socked, I had the same thing with my tower case when I moved a flat I discovered that the socket I was using had the ground and hot terminals switched.

This is a common problem, the power cable from the power adapter to your laptop is also usually connected to ground, if the ground is hot then you get shocked.