It's a difficult one. Let me say at the outset that I don't know
the answer but how does this sound:
Avoid getting rain drops directly on the camera and/or lens and be aware of the condensation that can occur when a cold camera is brought indoors. Don't worry too much about the humidity unless you are out in a fog (and that includes hill fog) where the air is pretty much fully saturated.
The reasoning is that these cameras are rated to work up to 85% humidity (manual page 170) and if you have a look at this New York Times article
you will see typical humidity levels for a number of places. Humidities can exceed 85% in the Caribbean at certain times of the year so technically the 400D/XTi would be operating outside its normal working environment.
As always, play it safe if you are in any doubt.