I trained for a 100 miler with a newborn. I've done multiple IMs and multiple 100s. The IM is a bigger time suck than training for a 100 miler.
that said, unless you have MASSIVE buy-in from the wife, I wouldn't even remotely think about training with a newborn. We had a very easy baby, so I can't speak to trying to train if you have a baby with colic or something. The positive of a newborn is that they sleep A LOT. I'd say the first week I didn't do any running. Week 2 I started getting a few runs in, and by week 3, I pretty much knew what the schedule was like, so I could time my runs around naps and such. I even supplemented with the bike trainer. Let my wife nap/take a walk/whatever. Put the kid in the bassinet, set up the trainer and rode while she napped, knowing that if she woke up, that was the end of my workout.
I also typically travel for work, so I'd load up my longer runs when I was on the road so that I had more time with the wife and baby when I was at home.
Its completely do-able, but your wife has to be 100% on-board, and you need to accept the fact that you will likely miss some workouts, or have less than desirable sessions sometimes.