Hi cpd, I had this trouble with my nikon 7900 and Canon 350D when on a cruise ship in the caribbean. The cabin was air conditioned and hence as soon as we hit the 90 degrees and 80% humidity outside the lenses and about everything else in the cameras misted and didn't clear for about and hour or so Also "fogged" pictures were produced long after the visible condensation had cleared externally.
For what its worth I got around this problem and used my wifes hair dryer with its diffuser attached and set to the low heat setting and warmed the cameras up before placing them in their cases prior to leaving the cabin and going outside .It worked a treat.

Regards Brian