Obsolete: With the Design Support Library announced during IO15 comes the CollapsingToolbarLayout which is much more powerful, elegant and easier to use. Use it!
It's like an Android ToolBar reacting to a scroll listener, but not quite.
This is a sample showing how to make a list
headerView looking like it is part of the
ToolBar and fade out its content when
the user starts scrolling.
I wrote a blog post explaining each step of the process.