If I was doing something in the house which meant I couldn't keep eyes on him constantly, I kept him in a playpen till he was big enough to just bust it apart to get out of it.
Then we had to figure out the gate thing -- we had a really tall one, about 4 ft tall, that's the "pinching" accordion kind... that gate terrified him so much he woudln't even come near it!
We had our own kid-vs-PC moment when he was about 3... to try and get in on the 'smart kid' movement, we let him sit down at the old Tandy after I got a Gateway... he decided to lean both arms on the entire keyboard... which confused the poor Tandy so much, it gave the machine-code equivalent of the BSOD, and it took me 2 yrs to figure out how to reset it.
Andria