I'm thinking it's cabbage worms or loopers. They are little green caterpillars that are the bane of brassica growers everywhere. They are suprisingly difficult to find but if you look near the stems of the plant you should find them. Judging by the size of the holes the caterpillars are still young. Try spraying with Bt or pick off the caterpillars if you can find them.

Rodney

Thanks, Rodney! What is "Bt?"

Bt is short for Bacillus thuringiensis. It's a type of bacteria you can buy that kills caterpillars. You spray it on your plants, the caterpillar eats the plant and the Bt, the Bt destroys the caterpillar's digestive tract (not sure of the actual science behind it), and the caterpillar stops feeding and eventually dies.

Rodney

Ah, thanks for the clarification and information. Pests, here I come!

Regarding whether to eat the holey leaves. I would certainly eat the one on the right but not the one on the left. Nothing to do with the holes but the left one just looks too old to be palatable. If those holes are representative of the type of damage you're getting I personally wouldn't do anything except give the plants a good shake and maybe a blast from the hose.

I don't know what Bt costs in the States but here it is extremely expensive and almost impossible to find for sale. I don't know why. It has to be ordered from the US and the postage at least doubles the cost. I wouldn't bother going to that amount of effort unless I had a massive problem and my livelihood depended on a perfect crop. A few holes are par for the course and will do you no harm.