Is there a way (out of the box) to know which Tridion editor/user has modified a Smarttarget promotion?
I've set the smarttarget log in debug, but I can only retrieve the last change date, not the user.
No. Unfortunately it is not possible to retrieve info which user edited (or deleted) smarttarget promotions. The issue with them is that they are not stored in tridion but in Fredhopper in business.xml file. SmartTarget is extention which allows you to edit them in Tridion easily.