I have taken my theatre training as an SM and applied it to my job at the ballet company -- I attend as many rehearsals as I can, take notes on times as the choreographer works (so he doesn't have to search through the CD to find the right place, and then start in the wrong place, thus wasting rehearsal time, etc.) Often he has told me that although he knows its time consuming, rehearsals are so much more efficient that way. while I don't take detailed choreography notes (worhtless; that's what video is for now!) I do take notes about entrances and exits, significant moments in the choreography (like a big lift, for instance). I actually find it easier to call a ballet than a play -- you know (for the most part) that it will always be the same. The music won't change; there's no forgetting lines, no pregnant pauses, no times when actor decides that they're going to ad lib...its always consistent (although there are the times when dancers decide they don't want to exit just yet, and you call a spot out cue when its always been called, only to see the dancer still peeking out of the wing and out of light!)
Yes, ballet is an entity unto itself, but in many ways more similar to theatre than one would think. (or at least, I make it so because as my director says, its much more efficient that way!) I know the show better, the director knows they can trust me, everybody is happier.
ASMing for a ballet involves more aid to the SM than in theatre. In theatre, often the ASM is put in charge of one area (props, the crew, wardrobe, etc.) and works somewhat independently of the SM. I rely on my ASMs for ballets to tell me when dancers are clear from the stage so I can bring up the next light cue or bring out the grand for bows, or let me know if a dancer is still under a piece so I don't bring it in too early. They don't have to read music, but they absolutely must have some knowledge of ballet and music terminology -- so when I tell them to inform the dancers we're taking it from such and such a place, they have some idea of what I'm talking about. Sometimes I rely on my ASMs to give dancers cues -- for example, if I have to take a cue off of their entrance, I have my asm cue them so I know when they're entering.
The ASM patrols the other side of stage -- so if I'm calling from stage right, the ASM is stage left making sure everything is running smoothly. If a dancer gets injured, they make the run to find an ice pack or an EMT if necessary.
My ASMs only work production week - I have them come and see at least one run throug prior to getting in the theatre so they know what's going on, but they don't need to know the show as well as an ASM for a play or musical. Esp. if you're working in a union house where you don't have to be in charge of the crew.
Jenk -- pm me if you want to know more...I'd be happy to answer further questions!