I can't speak for all, as I'm not an expert in all, but Ikelite, arguably one of the largest 3rd party manufacturers of housings for P & S cameras, are actually made from Acrylic. Basically plexiglass - which has many, many better features for underwater purposes than a polycarbonate material such as Lexan. In fact, Acrylic is used for many aquariums, swimming pool viewing windows, and even submarine windows. Here's a link to a decent article on it :
http://www.hydrosight.com/acrylic-vs...ve-comparison/
It does have it's downsides too, don't get me wrong, but it's a superior product for this particular application. And unless you're going from extreme to extreme as far as temperature, and humidity goes, I've never had a single Ikelite housing form condensation on me. And I don't even use silica gel packs. In fact, Ikelite technical support suggest you don't need any sort of desiccant product.
As an example - the water in the photo I posted above was about 4C (40F), and that dive (if I remember correctly), was well over 1.5 hours long. The surface temperature when the shot was taken, was at least 25C (82F), and I had no issue.
One must be careful not to load the camera into the housing in the comfort of your uber-air conditioned hotel room, and then expect to take it on a dive in the tropical Caribbean, and expect not to have any issues though.