Unfortunately, very few camera models specify the amount of resistance as anything measurable.

Water and dust resistance, as specified by Canon, means very little. If it said waterproof and dustproof that would be a stronger statement. I also feel lawyers got involved somewhere in the writing of these things. For example, some Nikon manuals say 'resistant to dust and casual humidity'. Again, it has very little meaning.

The 7D falls among those cameras with vaguely specified resistance. Unfortunately, you have to try to find out. The 7D I used was subjected to snow without any problems. Actually, falling snow rarely is a problem even for non-weather-sealed camera as long as you wipe it off before you go into an environment where it would melt. I have no experience with a 7D under rain.

To be sure about your equipment's resistance you have to try it and even there, bring a backup. I used a Pentax K-7 in a shoot during a sand-storm and it worked perfectly. When I got to the hotel, I rinsed it under the tap for a minute or so to clean it. The polarizer I had though took a beating and I could hear sand in it between the rings for weeks after. It probably saved my lens though :) On the other hand, it took less than 15s for a Canon Rebel (not sure which model, probably XSi) to stop working completely in the storm. Canon asked for $150 or so to clean the sand out.