For me, I've decided she'll have my last name. (Big surprise coming from me, eh?) Even though I don't have that last name right now, I probably will change it or the divorce will change it. (I still don't know if divorce automatically changes names or if they're two separate processes that must be done.) So she and I will go for a few months with different last names.
Actually, this is something I need to think about. . . because I could start the name-changing process now.
Anyway, my feeling is that even if we get back together, the different last name situation will just be a "scar" that our family will have. Or he could take my last name! The funny thing is we actually talked about that on and off during the marriage because my last name is so much more interesting than his. So maybe we'll all be under my name. Or maybe we'll switch to a hyphenation.
But to me the worst that could happen is that I go back to my maiden name, she has his last name, and then he remarries. Then he and his new wife will have my baby's last name! I don't like that. That really pushes me to ensure she has my last name.