Is there a way to clear the pop up notifications I get from changing the volume, brightness, dropbox updates, etc...?

Sometimes I have to sit and wait for them to go away so that I can continue to work, as they are blocking something I want to click or look at.

I know this question might seem a little naggy but it does indeed lower my productivity sometimes. I don't mind getting the notifications, but I do mind not having a option to clear them.

Any idea how I can do that?

I am using Ubuntu 11.10 (Gnome)