"50mm f/1.4 is what you want"
And a nice 400Mpixel camera so you can get those close crops when required 
I would agree with PhilHannah on this. I only have access to a Pro1, so often find it rather a challenge to take decent stage photos. What I have learned, is:
- If possible, shoot RAW and expect to mess around with the colour balance later. When I shoot JPG I often find it hard to pull the people out from the set; the colour cast is just too strong. And of course in most shows it will vary throughout the show (remember that the dress rehearsal is normally a rehearsal for the techies too, they might not want to change all their lighting plots for your benefit)
- Often, even with my ISO400, I can get away without flash for well-lit scenes. With practice, you can hold a camera pretty steadily up to your face for 1/15 of a second
- Dancers are a problem. They move fast. So do you use flash and lose the effect of the stage lighting, or do you have blurs where there used to be hands or feet? I don't have a good answer to this one
- Spotlights are another problem. It seems to be the fashion just now that whenever anyone starts singing, the spotlight gets hammered on to them. I have LOTS of photos of a chorus surrounding a white flame that once was a singer. Youcould spot meter, of course, but then you've got a properly exposed person and a mushy grey mass around them. That's where the flash can come in handy
FWIW, I've got a few sets of amateur musical stage photos at
http://www.flickr.com/photos/atp/
. The pictures there fall into two categories - some where I've dumped everything so that people in the club can get what they want, and others where I'm trying to put only the decent ones. The latter can normally be recognised by having
many fewer pictures!
Good luck, and please share the photos you get. As an avid amateur musical performer myself (currently doing many fewer shows because I have young kids), I
love looking at show photos and dreaming that I was on stage!
regards,
/alan