FMLA (federal law) protects you so your work cannot fire you during maternity / paternity leave, although you may have to jump through some hoops to protect yourself (like submitting "proof of birth" to a third party claims administrator if they require it, which is unbelievably frustrating). However, I do not believe it requires your work to pay you for your time off. Mrs. Catastrophe recently did this and had built up enough sick leave to get paid for the full amount. Her work told her that they are apparently required to pay her sick leave along the amount she were making some months leading up to leave.
I do not think there is any reason to tell your employer that you do not plan on returning after taking leave. I certainly do not think you are required to do so. That said, you are probably protected enough that you could give 2 weeks notice prior to the end of leave if you wanted to be courteous about it.
