The fogging problem is very rare on DSLR housings, and mainly a problem with the smaller compact housings. The reason being that there is much more air trapped in the larger DSLR housings and the minor humidity that finds its way through is negligible and does not create fog.
DSLRs also don’t usually heat up as much as compacts.
Placing one of these inside the housing is recommended, just in case you have a minor leak due to a hair in the O-ring:
Ran is a professional photographer for over 14 years. His passion for scuba diving and photography has pushed him to combine his profession and hobby and become a professional underwater photographer. Teaching is one of his greatest passions and over the years he has shared his experience with many divers and aspiring photographers. Along with his wife Danielle, an experienced Scuba Instructor, they have founded Dive and More, leading dive trips and UW photo workshops all over the world. Ran is also an electrical engineer and an avid internet marketing specialist. Visit his personal portfolio at www.ranmorphoto.com.