Specialties in the AF Nurse Corps include pediatrics, neonatal, ER, critical care, med surg, flight nursing and psych. I'm sure that there are likely more, but those all come to mind pretty quickly.
I think they want you to have a least a year of nursing, but I think that it's a good idea to have it...if they require it or not. Actually, if you have less than a year, I think they just send you to NTP to get your feet wet. It's essentially a long, detailed orientation to AF nursing.
If you talk to an Active Duty nursing recruiter, then you'll be active duty. If you talk to a reservist nursing recruiter, you'll be in the reserves. In the reserves, you'll have several months of active duty time for schools, training, and "seasoning"...similar again to an orientation type situation.
Is it a good idea? I love it. Is it for everyone? Not by a long shot. I can tell you after 8+ years of nursing, all of it critical care, flight, and ER/trauma, I experienced my most rewarding moment last week bringing home our soldiers, sailors, airmen and Marines...I can't even put it into words, to be honest. Amazing isn't even close!