Thanks for the responses guys! Gothmum I hear you on the spelling factor, I *thought* I wouldn't have to spell the Bear's last name (I always had to spell my maiden name), but alas people seem to need that!
I think what got me thinking about this was some friends of ours who are married but bubs both have mum's last name. And mum and her two sisters all have different last names - ie one has her mum's, one has her dad's and the other has both! Also I was looking around my mum's group and wondering what the baby's last names were... and I'm still too shy to ask!