Yes, dogs can have popcorn in moderation. You should however note that it should be unflavored, as some flavoring can give dogs upset stomachs. If you do give your dog some popcorn you should not give it any more than a couple pieces as it is unhealthy.
Although dogs can digest popcorn, it can lead to skin problems. Popcorn is a junk food, and it should not be given often. There are many cheap dog foods that will use corn in them as a filler. These foods should be avoided as they can lead to skin irritation.
Like all foods, dogs can be allergic to popcorn. If you have never given your dog any before you have to make sure that your dog will be ok with it. Start by giving one kernel, and see how your dog reacts to it. If he or she is fine you can give them a little more.