Our oldest (3 at the time) played up BIG time when little brother was born. Now, our circumstances were a little different to most as I was bedridden for a month after his birth, so from Ds1s perspective I was in bed, and baby was with me, and he was downstairs doing his regular stuff, looked after by granny, auntie, daddy, etc.
When eventually I got well enough to "rejoin normal life", and got an au pair, we found that it was better if I handled DS1, and the au pair carried ds2 around. The oldest child needs mummy more than ever when there is a new baby in the house, it is the worst time at all to "put him aside", so it is useful to remember, that baby can wait a little for a nappy change, he can cry a bit, and your oldest MUST feel important.
However, if you are planning to send him to daycare, or nursery, or even increase his hours at the nursery, it might be a good idea that he starts these activities before the baby is born, so he doesnt feel pushed out, iyswim.