You can see an early version of this script in action in Workbench's Pump Up the Jam tutorial at the 4:09 mark.
StackIt is an After Effects script that allows you to stack layers into any sized grids, staggered grids, rows, and columns using selected layers. It'll also duplicate as needed to fill if you want it to. Combined with Workbench's Radial Delay script, you can make some pretty interesting things with StackIt. It's already saved me quite a bit of time. I hope it can do the same for you.
This download comes with the script and a user guide.