A few more thoughts...
That 100 f/2 lens is really nice. My experience, however, is that shooting moving subjects with a wide aperture leads to blurry results too often. Personally, I'd want more depth of field. Without actually being in your situation, I'd guess f/4 would be a lower limit. It's too bad the Canon teleconverters don't work with the 100 f/2, as that would give you some reach in return for aperture.
If you need a bit more reach and want to stay fast and cheap, the 135mm f/2.8 soft focus is impressively sharp when the soft focus is set to zero. I saw one on eBay for $360 refurbished.
If you have a lens hood, use it. It'll protect from physical damage as well as lights aimed at your position.
It appears that the XSi is capable of serviceable images at ISO 1600. That's good news, and you may need it. In my experience (with a different camera) Lightroom 3 does very well with noise reduction.
Whether you use manual or auto exposure, make sure you account for the snow being so white. I usually over-expose as much as I can without blowing out the highlights. Of course, night shots might be different.
Good luck. Have fun.