RN practicing out of scope?

You have to check with your states nurse practice act. Usually with training it's fine. I have considered taking a course to become certified myself, but I don't believe I will use it frequently enough to keep my practice accurate. ( same reason when we go to a code that requires we use ACLS we act as recorders). We have an out patient nurse that would probably make more use of an ultrasound certification. Our hospital has done classes, but just for ultrasound guided IV starts. It's like many things in nursing that require extra training and then you can do it, such as the nurses that insert PICC lines.

I imagine it would be up to your facility and their policies

I think it depends on hospital policy. At my facility, ultrasound certified nurses can use ultrasound to check for position. Our policy states presentation will then be confirmed with a second u/s certified RN.

I Imagine the MDs do their own sono upon admission? That's how we do it, an MD always checks for position on admissing, even for scheduled c-sections. RNs at my job use the ultrasound sometimes w/o "clearance" from MDs to help find the heartbeat on difficult babies (preemies, twins) to help with EFM. I'm pretty sure RNs can use the ultrasound in office settings too, with the right training. Not 100% sure though.