need some advice on lens purchase...

What about the 85 f/1.8, that seems to get great reviews:
http://www.fredmiranda.com/reviews/showproduct.php?product=32&sort=7&cat=2&page=3

Also, on a 1.6 crop camera, the 50 f/1.4 is a awesome portrait lens, yields about an 80mm equivalent.

EDIT: Wait...Canon doesn't make an 80 does it?

 

I have the both the 50mm f/1.8 and the 85mm f/1.8 ... I love them both, but find myself using the 50mm one much more often. That's due to the fact that most of the pictures I take are indoors - and find myself backing up too much with the 85mm one. The 50mm is also smaller and lighter, so makes for a smaller camera if I'm going somewhere and only want one lens. (I think the f/1.4 version of the 50mm lens is a bit larger/heavier though)

That said, if your main goal is to get something like head/shoulder portraits, you might consider the 85mm one. You'll get a little better looking bokeh (blurred out background) since it's a longer lens.

Here's one taken with the 85mm lens:




This was taken with the 50mm:




and I'm 90% sure this was taken with the 50mm also:




So, you can't go wrong either way with those. I'm not familiar with the 100mm one you mentioned so can't really comment on that.

 

well I don't own the 85 or 100, but I have the 50mm 1.8, and if you can get a 50mm 1.4, I highly recommend it. Awesome for portraits.

here's some I've taken with the 50mm 1.8.
the first two are 35mm film, the rest are with a Rebel XT

http://www.deviantart.com/deviation/48695642
http://www.deviantart.com/deviation/48695701
http://www.deviantart.com/deviation/42641417
http://www.deviantart.com/deviation/41306930
http://www.deviantart.com/deviation/41306670
http://www.deviantart.com/deviation/40890521
http://www.deviantart.com/deviation/40512040
http://www.deviantart.com/deviation/39892122

You can get the 1.8 for like $80.

 

I'd go with the 85mm. The problem with the 50mm is that you can get perspective distortion on head-and-shoulder shots, particularly with children. In addition, the 85mm has USM focussing, whereas the 50mm 1.8 does not. At f/2.8, the 85mm is among Canon's sharpest lenses - better than my 200 f/2.8L at any aperture. If you could swing the extra $80, get the 50mm for torso shots or for two or more persons in the frame.

 

My opinion is that the 100mm on a 1.6 crop will be too long for indoor portrait shots and you may sometimes find it short in some outdoor sport situations. The 85mm would be better for indoor portraits but not very useful in sports.

My advice is to get the 50mm f1.8 for now which is really really cheap and save your money, so you can get a nice 70-200mm zoom for sports later on. This way you have everything covered!

 

I have had all of these lenses and for the Rebel, I agree with this statement.
Get the 50mm 1.8, it's cheap and best for portraits.