K.,
My little guy is doing this too. He is fine when we put him down to play but if we put him in his playpen he gets really mad and screams. He also screams for attention. We have tried the "no" (which we later switched to "all done") method and ignoring it, he still screams this high pitched ear piercing scream. I think it is just an attantion getter. That and since they don't have words, but have oppinions, they are just trying to get their point across the only way they can - by crying. Incidentaly, my little dude was born June 25th, 2008. :)