I don't think you can do it in general. I am looking at the source code for Android 2.1 launcher (in packages/apps/Launcher directory), and there is a method (Launcher.setScreen()) that they use to change the active screen, but it is only called internally from Workspace.java in three places: at initialization, to set the default value, on restoring the instance state, and when scrolling.
It is possible that the particular launcher application you use allows external intents to set the home screen, but I wouldn't know how to track it down without access to the source code.