Yep - again - I will say WOW! VoiceThread is an amazing tool for teachers and students alike. I have just created a mini assessment piece that I could have used in my students two weeks ago when we were talking about the Titanic disaster. I uploaded some photos from Wikipedia that I showed the students, and asked them (via typing) questions regarding what they had learnt. The students could then answer the questions in their book, and then respond to the pictures with other information that they know about the sinking of the Titanic.
This is just one simple way to use this technology. There would be countless others, and they would all have the same aim - to get students thinking and working collaboratively, one of the engagement theory's components.