What blocks the radiation (at ~2.4GHz) is the metal all around the cooking cavity. The holes aren't actually a problem: as a rule of thumb we don't worry about shield leakage once the maximum dimension of a hole (diameter, in the case of a round hole) is less than 1/2 wavelength. 2.4GHz has a wavelength of 12.5 cm. Leakage does not happen through those holes.

(It does happen through the gap between the door and the rest of the microwave, but there's some neat design work to keep it from getting too bad.)

Of course it leaks, though. It leaks so much that when I was working on sensitive RF receivers, I could see when microwaves were turned on for lunch hour in other parts of the building. It's the leakage that makes it interfere with wifi. Don't worry though, you absorb more RF energy through the wifi in your devices and through your cell phone.

Wasn't implying it leaks by the holes :) just that nobody (i didn't use to) think it was a sheet of metal there only

Interesting fact: a relative of mine has a pacemaker, and can feel leaky microwaves (I guess there's some voltage induced on the edges of the device, and some nerves pick it up). It turns out that almost all old microwaves (older than 5 years) are leaky, and she has to leave the room. Most new microwaves are fine.

Of course there's a percentage of radiation that gets out, that's how you can see your food spinning round.

I'm fairly sure the holes are sized such that only certain wavelengths can leave, but that could be nonsense.

It's not nonsense

Of course, you can block/scatter visible light that way, but it's awfully difficult (since the holes would be tiny)