I am 30+1 with first child, and not yet married to DP - got a bit hijacked by pregnancy! 
My DP has two children, he was also not married to their mum at birth of first child - they married three months post birth, but the child had his surname from birth.
Basically he expects the same to happen again, which I am quite against, mostly because I will be the one taking the baby to all of the appts and would be a bit odd for us to have different surnames!! We're not planning to get married until next winter, so it would be a whole year of having a different surname to my baby - although I suppose it's going to happen to one of us either way...
I think his issue stems from fathers having less rights if not married to the mother, although from what I gather rule changes in 2003 mean this is not the case if you register the birth together (I am assuming this literally means you both go to register the birth - who knows!)
Also on a more petty note when my Step-daughters are here, there would be 4 people in the house with DP's name, and just me with mine!! Sounds trivial, but feels important...
So I have pretty much decided that the baby is having my name, which I think is fair - will break it to him in a few weeks I think. Nothing like a bit of procrastination 