One of the really clever aspects of Google Wave is the ability to create private blips that reply to a parent blip.

This is very useful if you want to keep a separate note about a particular blip or if you want to include a co-worker in a side discussion without impacting the conversation still carrying on.

You can even invite an additional person from outside the main wave who can comment in your private blip and view the parent blips content but not the entire wave.