There's a hidden function inside the default Cake parent model that checks for fields with the name 'created' and 'modified', and updates the values automagically.
Search inside /lib/Cake/Model/Model.php for the string 'modified', and you'll see lots of references to this.
I'm not sure how well this is documented, I came across it while debugging some funky saving behavior.
In my system I settled on the fields 'created_at' and 'modified_at' before I found this little gem.
EDIT: This is documented here: http://book.cakephp.org/2.0/en/models/saving-your-data.html#using-created-and-modified