In Moodle 1.9 we had a special admin page for this, but this doesn't fit into Moodle 2.0.
In Moodle 2.0, the way you want to do this is by adding a block to the front page (which most people see as the "top" of the system) and then make it sticky "all the way down".
The trouble is that there are two interpretations of this at the frontpage site level:
1) All the activities in the front page course
2) All the contexts in the whole site
My solution is to just represent these two modes in the block config screen and let the user know the situation and decide.
It's as simple as a menu with two options to replace the uneditable "This block belongs to" item and adding some help, so I won't bother with a mockup.