Not sure if your dance performance is over or not already, but I'll lend my experience...
I've rented lenses from
Calumet Photographic twice now. The first was a Nikon 70-200mm F2.8 VR, the Nikon equivalent to the Canon EF 70-200 F2.8L IS that
GTaudiophile had in his first post in this thread. The second was a Nikon 18-70 F2.8 (no VR).
The 70-200 I used at my little cousins church Confirmation ceremony. Flashes were allowed but I was way too far away for my camera flash to be useful, and I didn't want to be abnoxious. I shot handheld with a 400-800 ISO and got many nice shots. Given the lighting (wasn't dark but wasn't that bright either), the distance, and the normal use of my kit 18-70 (I think) F3.5-4.6 DX lens, I can tell you that a minimum of F2.8 is necessary if your going to be in anything other than brightly lit enviroment...unless you want your ISO to be so high as to have your images be grainy. Also, I used to own a Nikon 80-400mm F4.5-5.6 VR lens. While it's a really nice lens, if you don't have the light, it's much more limited than you'd think. The VR helps greatly, but, not really enough - too many missed/bad shots.
The 18-70 was also a really nice lens. You can just tell pro lens from consumer grade..they're larger, heavier, feel more smooth, and the low F numbers on them make for great opportunities.
I'd second the notion of figure out where you're going to be sitting in relation to the stage. if you sit close and bring a 70-200, you're going to have too much zoom. If you sit far back and bring a 50mm prime, no matter the F you're going to be F'd. Try and figure out where you're likely to be sitting, and then judge what lens you'll need from there.
If Canon had like a 18-70 F2.0 or something, I'd say that'd be a great all around lens..a little more ISO than a F1.4 or F1.8 prime won't kill
that much, and it'd give you some more flexibility.
Chuck
P.S. My renting experience was top notch at Calumet Photographic...great rental prices, and the equipment was in superb condition.