Well, depending on the location 100mm might be too long. If it's a small venue, the 24-70 2.8 on FF/1D3 will probably be better. If you have more room to work with, the 100mm might work out for closeup shots. If it's really a bigger venue and you have to stay to the back, I'd rent the 70-200 2.8 IS.
Depending on the type of dance, you'll either need to really crank the ISO to get fast enough shutter speeds (e.g. 1/200+). If it much of it is slower, which going to rehearsals really will help, then you can see what parts you can slow down the shutter, and when you need it as fast as possible. You'll also find that, with the 5d2, you'll often need to pre-focus where the dancer(s) will be and then snapping photos at the correct timing. The 7D and 1D3 AF is much more capable for AF-Servo tracking.
Also during rehearsals, take note and remember the most dynamic sequences such as lifts or other big deal movements so you'll know when they are coming up during the performance, and do your best to get those as well.
Last, shoot RAW. Trust me, if the lighting is even a little bit odd or changing, which it will be, soooo much easier to get the color temp correct in post.