Are there any circumstances under which a dad can bring up the subject of "baby weight?" What would make a concern over a new mom's weight legitimate? And if there are legitimate concerns, how can they be addressed without doing serious damage to the relationship?

What if this is an impediment to marital love life "getting back to normal?" Could that be considered legit?

Tough call...

I guess I've been lucky.... My wife used to go to the gym regularly before the baby. So she was well aware of her size/weight. I shared my progress at the gym with her. "Hey, I ran for 30 minutes today", or "My arms are sore from weights this week". She would get jealous, and say "I wish I had time to go to the gym."

So I gave her time. Started coming home from work and saying "Get to the gym, I'll watch the baby." A month later, she's going to the gym 3 times a week and bragging about getting back into her pre-pregnancy 'skinny jeans'. She's still got 4-5 lbs of baby weight, but I wouldn't know if she didn't tell me.

I think the biggest impediment to 'marital love life getting back to normal' is exhaustion. That, and having to re-define 'normal'. Before baby, mornings used to be the best time for us. Not a chance now.

I wouldn't bring up 'baby weight' for the same reasons you outlined. But sharing fitness goals, or investing in a Wii Fit, might work.

I think if the mom isn't seeming to do anything about the baby weight months after pregnancy and isn't making any plans to, then the dad could say something. Gently of course. Same goes for dads and sympathy weight.
I don't really know a way of going into that subject without hurting feelings, though.