The concept of "Privacy" means to me that the average user is aware of how their profile, their content, etc is going to be used by the host.
There's a difference between the perception of your feed being public, and Twitter selling your feed data to a corporation to utilize for targeting, advertising, reselling to employment agencies in the future long after your stupid teenage profile is deleted, whatever.
When a solid 10% of the users are kids (and a much much larger percentage is entirely clueless) it's worth questioning and the people that do know what's actually going on have a responsibility to ask questions.
That's my best attempt to get fired up about privacy on twitter, you forced my hand - oh won't you please think of the children?
Ref twitter age -
http://royal.pingdom.com/2010/02/16/study-ages-of-social-net...