Taking a camera / lens between sudden temperature extremes can cause condensation issues. We had a good discussion of this not too long ago ...
Can cold temperature & condensation damage my camera or lenses?
No - the 5D isn't particularly weather sealed. The 5D Mk. II has vastly improved weather sealing, but I would put this into the "Nice to have as added insurance" category rather than the "Cool - lets so shooting in the rain" category.
Canon 1D series cameras are the only ones that are fully weather sealed (I've had mine pretty wet at times - didn't bother it in the slightest) - however you need to make sure that the lens is also weather sealed, which means using one of the weather-sealed L-Series (contrary to popular opinion, not all L-Series are weather-sealed).
Does this help?