I tried to summon a zombie during the day with an iron helmet using this command:

/summon Zombie ~ ~ ~ {Equipment:[{},{},{},{},

]}

And what I got was a summoned zombie, but with no helmet. And it also burned in sunlight, which tells me that the helmet was not invisible or not rendering or something. I repeated this with a skeleton, with the same results. All the other slots (weapon, feet armor, leg armor, chest armor) don't work either.

Then I tried to summon a regular zombie, but use the /entitydata command to give it an iron helmet using this:

/entitydata @e[type=Zombie] {Equipment:[{},{},{},{},

]}

The game gave me a message that the entity data WAS changed, but I noticed no difference on the Zombie.