How many lands do I need for a Commander deck?
Because of the slower pace of play in EDH/Commander, it isn't as drastically important to hit 5-6 land drops in 5-6 turns.  In addition, the number of "mana rocks" (mana-generating artifacts) and dual lands/mana filtering/land fetch cards are usually higher.

Considering that, the 40% lands rule is still fairly close to normal (lightened obviously for the reasons above).  I've run 34-36 land decks successfully, but usually you see 37-40 with some of the previously mentioned accelerators.

Abnormal decks (with huge numbers of accelerators) will run as few as 32 lands (the equivalent of running about 13 in a 40-card limited deck), but I'd recommend starting around 40 and cutting around one land per three accelerators (if you're trying to cut land, and not running very high-cost spells). 

Do also remember that running fewer lands with more mana rocks leaves you open to blowouts when someone wipes artifacts with Austere Command, Merciless Eviction, Shatterstorm/Creeping Corrosion, etc. - you don't want to be stuck on four lands when your couple of Signets and Thran Dynamo get wiped.

As an example, the Devour for Power Commander deck shipped with 40 lands and four mana-generating artifacts, but it is fairly heavy on costs.