Gnome users know F-spot, a photo management software for Gnome.
I used to use F-Spot for a while and my photo library of almost 13 000 photos is organized with the following directories: Year/Month/Day.
Importing all of this into Picasa could be as simple configuring Picasa to watch the old F-Spot photos directory.
However, I wanted my photo library to fit Picasa's naming conventions which is one directory named Year-Month-Day and all the photos of that day inside.
What follows is the code of a small Python script I wrote to do the job.