Yep, the baby can have any last name. Actually, I don't even think the last name has to even be a 'family' name... you could just make one up as far as I know (but then... I don't REALLY know lol, so best to check it with a professional!).

Depending on which state you are in depends on what happens with the birth certificate.
QLD has passed the act which allows both mums on the birth certificate, but not yet proclaimed it, so it isn't technically 'in' yet. NSW allows both mums on the birth certificate. So does WA I believe. Not sure about the other states. I'm pretty sure SA is still in the dark ages about it...

As for the donor, well for me I just wrote 'unknown donor' where they wanted to fathers details and that was it, the birth certificate came back with nothign written there, just a blank space. Can't wait until QLD get their act together and my partners name can be there...