One of my bio professors many years ago brought this up as a point of discussion on a moral basis, although the question was framed more as "Is surrogating okay". It lead to a very interesting discussion, but generally boiled down to two primary questions:
1) why is the surrogate willing to do this;
2) why is the mother seeking a surrogate
In the case of 1) the problem is whether the surrogate mother is 'renting' their body as an act of desperation (money), with intention to perform criminal action (extortion, kidnapping), or as a more intrusive part time job.
In the case of 2) the questions become somewhat more difficult. If the mother seeks to employ a surrogate due to emotional, financial, or time problems the question must be asked whether this person can ethically be considered fit to act as a parent, as there may be no reason to believe these personal issues will be solved by the time the child has arrived. If the decision to employ a surrogate is based simply on comfort then there should be no problem, but unfortunately there is still more to deal with.
In terms of liability should a miscarriage occur or non-genetic disorder appear in the child, is it the responsibility of the surrogate mother? In these cases is the surrogate mother still entitled to pay despite the failure to deliver a working 'product'? Should the surrogate mother be allowed to forfeit her pay and choose not to deliver services rendered?
I can only imagine the size of the contracts one must sign in order to enter into this kind of agreement, especially given the nature of the 'product' as a living being.
