I think i found a way to do that:
decompressing Launcher2.apk (trebuchet Launcher app) i found an interesting file named
default_workspace.xml under /res/xml folder of the app.
<appwidget launcher:className="com.estrongs.android.taskmanager.widget.TmWidgetProvider" launcher:packageName="com.estrongs.android.taskmanager" launcher:screen="2" launcher:x="5" launcher:y="0" launcher:spanX="1" launcher:spanY="1" />
<favorite launcher:className="com.android.music.activitymanagement.TopLevelActivity" launcher:packageName="com.google.android.music" launcher:screen="2" launcher:x="5" launcher:y="2" />
with all the necessary to place widgets and icons on different workspaces and XY position.
Hope it helps