We went through this too. Ds is now over it. I think it's due to their increasing capacity for imaginative thoughts. I think you're right about the tv/dvd's causing some of the fear as well. I like the idea of talking about cookie monster, Elmo, etc. I'll have to try that if the monsters reappear!
Just an alternative viewpoint...we opted to not use the monster spray as we felt that would be sending ds the message that monsters are, in fact real if we have to use monster spray to keep them out. Instead, we found that ds responded to us saying, "monsters aren't real, they're only in books/on tv. we don't allow scary things in our house anyways...mommy and daddy wouldn't let any scary things in". I know some people have suggested putting a sign on their bedroom door saying, "No Monsters Allowed" or something to that effect. Again, we chose not to as we felt it was saying that monsters are real.
We also made sure to spend lots of time in his room during the day to make it a positive, happy, fun place to be. Having a teddy bear (or 2, or 3, or 4.....like our ds!) helped too. We told him to cuddle with his bears if he was scared.
HTH!