What I can seem to gather info wise is that the final episode of the second season is a cliffhanger and you dont know if she will survive???
My guess is that she will stay on at least part way into series three.
She has earned alot of credit within the role and the chemistry between her and Tennant has been noted as electrifying so I reckon we will see her in most if not all series Three.
As for replacement, I think we should go for a reltive unknown person, but it is mainly the writing that brings the characters to life so I have no worries about who will replace her. Mr. Davies isn't going to allow Dr. Who regenrate into what the show once was. This is his baby and he wants to watch it grow.