As far as I know, at the hospital that I work at now, the OB nurses do not float to the med-surg areas UNLESS they have been cross-trained to those units, and they don't have to be cross oriented unless they WANT to be.
At my old hospital, they floated the Post-partum RN's and Med-Surg RN's to each others units. When an OB nurse came to my floor--she never took an assignment and basically functioned as an aide--answered call lights, got vitals, did blood sugars, etc. What made me upset was that when I was floated to the post-partum unit, I was expected to take a full assignment, no objections allowed. I really enjoyed getting floated there, unfortunatley, each time that happened, I only got to stay for 4 hours and then had to go back to my unit