"i'm an avid backpacker..."
Yes, it is important with this in mind. If you spend the bulk of your time shooting outdoors, it is essential, IMHO.
Non-weather-proofed bodies may function in the weather once in a while...however, issues may or may not be immediately apparent. After time, a non-weather sealed body's internal electronics may show negative effects after spending time in rain, sleet, snow, ocean-spray etc. IOW, it may not necessarily be an immediate effect, but rather a long-term moisture building inside detriment.
If you spend a lot of your time shooting outdoors, the extra money will buy you peace of mind...but, keep in mind, it's not just the body, but your choice of lenses as well...as that is where one of your weakest seal points will be. The more spendy, weather sealed lenses have gasket seals that are made to withstand moisture, sand, snow, salt, etc.
After shooting for twenty years in Alaska...and I do a lot of backpacking, fishing, hunting, and other outdoor photography...I will not buy another non-sealed body.
That said, I don't think any of the manufacturers claim to have "water or weather proofed bodies" but rather "Weather or Water resistant." I shoot with a 5d2 like the previous poster and have been caught off guard in weather and it's been fine (for a year)...but if I know it's going to be crappy, I'll put a sleeve or bag on the body and lens. They're cheap
I just got rid of my 1d3 in preparation to purchase a 1d4...and, IMO, if you leave even a 1d series out long enough, it WILL fail! Same with the Pro series Nikons...including the D3.
Good Luck
J