current leakage in a laptop
The problem is that in the absence of any other factors, the secondary of a power transformer will tend to float at half the line voltage, because of the leakage current through the primary-to-secondary capacitance. It really doesn't matter a whole lot whether it's a line-frequency transformer or a high-speed switching transformer, although the capacitance in the latter should be somewhat less.

Normally, this leakage current is conducted directly to ground via the third wire in the line cord, but in a 2-wire cord, this is not possible.

Since the impedance of a capacitor drops with increasing frequency, yes, it could be a concern that the leakage current could be much higher in the presence of fast transients on the primary side. A surge protector inside the power supply will limit the differential voltage across the transformer primary, but it won't have any effect on common-mode voltage offsets. A common-mode choke will have some effect on the latter, but probably not enough to be considered a safety feature; it's mainly there to prevent switching noise from inside the power supply getting back out onto the line.

An electrostatic shield inside the transformer that's tied to the Neutral side of the line input could reduce the leakage considerably, but with nonpolarized plugs (and the possibility that the wall socket is miswired) there'd be a 50-50 chance that it would make the problem worse rather than better.