WRT Portsmouth, militarymd has it about right. As a CA1 I haven't seen rotating students much, so I figure they must be 1-on-1 with staff or CRNAs.
There are 18 Navy anesthesia positions each year, 6 each at NNMC, NMCSD, and NMCP. For my class (entering 2006) there were 33 alternates - I don't know how many of them were interns vs returning GMOs. Last month's match (class entering 2007) had fewer alternates listed - I'm not sure if that's because there were fewer people interested this year, or if interns were discouraged from submitting an application, or what. There is no categorical internship; everyone must apply as an intern. Everyone taken in recent years has completed a GMO tour. It's more competitive than it was a few years ago, but I think almost everyone who does their time as a GMO gets a spot.