Sorry, forgotten the poster who asked, but yes, I do think that it's tough luck if a surrogate changes her mind, if she is not the biological parent of the baby. I truly believe the parents are the couple whose embryo used.
I think this is one of those things that you can maybe make logical sense as a kind of thought experiment, but the actual brutal reality of it - forcibly removing a baby from a woman who has recently given birth; would you restrain her to the bed? Or arrest her if she refuses to let go of the baby? Maybe you'd get the hospital staff to take the baby while she sleeps? - is pretty indefensible.
Also, my (partial and non-expert understanding) is that the idea that the child would only have two biological parents here is very problematic, as the more we know about the importance of the uterine environment the more we discover that it has life long effects. Women who use donor eggs obviously don't have children who carry any of their own genes, but evidence is beginning to show that they do affect how the child's genes act:
www.bionews.org.uk/page_95263