I've just gotten a second child, and what I've learned is that you never want to try shooting them while they're awake, it doesn't work! Make a comfortable place, such as a soft rug or something, build and set up the scene long before the baby gets there, let everyone else take their pictures, and wait for the baby to have eaten, given up, and fallen to sleep. That's when you place the baby in a comfy chair (lying down of course), then you get those nice supports which frames the baby better (armrests) and you want to not use your 580? I say use it for all it's worth, not directly to the baby of course. Set your camera so that the flash only fills slightly in for those soft light shadows, place the chair so you can bounce the flash into a light wall before it bounces off the baby. That way you get soft, not directly on -light, without wireless and diffusers.
Another thing, have it warm enough so you can shoot the baby with only diaper-pants or nothing on, be sure not shoot so that you can see any of the intimte parts, because no one wants to see that, EVER. Plus it's very disturbing and freaky, nuff said.
Use a an elf-hat or some light colored blanket or something to add some light color, make sure the color matches if it's boys or girls.
Try not to pose them too much, they look very peaceful and easy to shoot just as they lay sleeping.
Here's a shot I did of my daughter. It's shot with a double umbrella-setup, wireless, but similiar effect can be done bouncing off the wall. Shot with 85mm at mfd. I like to tune down my flashes, and use wider apertures, or faster shutterspeed to get the shallow dof, and softer overall feel. Hope this helped.http://photobyviggo.com/random/mariell23.jpg
and one more, same setup:http://photobyviggo.com/random/example.jpg