Find your nearest carrier's service center (the place that actually fixes the phones) and take it in. Mine's going back tomorrow. I use the Nova launcher, but every so often I find myself closing a program and being back in the native Samsung launcher. One press of the Home button fixes it, but it's annoying, and it could point to other problems.
At least gget it on the record that the phone was defective from the start. Then you have a strong case for a warranty swap if it waits until the warranty expires before it dies.