Hi Eagle. First post for me. Sole reason I purchased a Canon 1000D was to photo my kids' dance recitals & competitions; I'm a father of three. I purchased a Sigma 50-150 for that purpose and was unimpressed with my pictures at max ISO for that model; I didn't know then what I know now about photography equipment.
Two months ago I purchased the 550D and BLAM!!! What a difference. Still use the Sigi but now with higher ISO. 1600 minimum, but usually 3200 & 6400.
Having been a dancer myself, I know how to time the jumps and leaps. Best advice I can give is get a lens with a 2.8 (which I see you have) or better (I've used a 50mm 1.8 at rehearsals in a studio and I know the pros use the 70-200 2.8 at comps); NEVER use flash (a dancer's pet peeve), try to use manual focus for comparison (my eyes are bad these days so I rely a lot on AF), and learn how to time the shot. Oh, and post processing; i.e., noise reduction, is your best friend for shoots such as these.
Mike L. 7D gripped, 60D gripped, T2i gripped, 24-70mm 2.8L, 70-200mm 2.8L IS, 85mm 1.8, 50mm 1.8, Tamron 17-50mm 2.8 (non-VC), Sigma 30mm 1.4, 580 EX II, 430EX II, Pro Runner 450 AW. And a partridge in a pear tree.