Radial Delay is an After Effects script that takes selected layers and delays them in time based on their distance to the center of a composition. Check out the video above for more information on how it works. You can also check out the tutorial for the Radial Delay expression. While the expression is good, the script works much faster because it actually offsets the layers in the timeline so there's no additional calculation once you've applied the changes.
This download comes with the script and a user guide.