Whilst I did not carry my DSLR (Canon 5D MKII) on the Challenge I have carried it quite a lot on smaller trips, in fact, I have it with me nearly all the time.
The solution I find, for any camera, is to fit it into the smallest possible bag I can find because it makes it less baulky and I am more likely to carry it with me everywhere I go. For walking and backpacking I either hang it from the hip belt (because it has a belt loop) or to simply hang the bag in front of you, or a combination of the two. If you store it in your rucksack all of the time then you are unlikely to use it much and then regret taking it, but if it is to hand you will use it and subsequently enjoy the quality of the images compared to a compact camera.
If the bag has an all weather cover then that will keep most of the rain off and the camera will stay dry but if it is a particularly rainy day then you might need to put it in your rucksack in a drybag. Spraying the bag with the same stuff you use for walking boots helps stop rain soaking into the bag.
When I was climbing (via ferrata) in the Dolomites I used a Lowepro Toploader 75 (the old model) to carry my Canon 5D. I hung it in front of me and put the strap round my neck. I then had a strap that went round my waist to keep it steady while I was climbing, it worked a treat.
The 550D is very similar and lighter and if you did not want the video then I would recommend that but if you might do video, and video of yourself (it has a flip out screen), then the 600D is would be better. Also the 650D has just been announced which might push the 600D prices down.