From http://help.eclipse.org/help21/topic/org.eclipse.platform.doc.user/tasks/running_eclipse.htm:
Use the following command-line argument:
-data your_workspace_location
For example,
-data c:\users\robert\myworkspace
you can also use UNIX-style relative path names such as
-data ../workspace
even under Windows, in case something doesnt like colons or backslashes in parameters, like Jumplist Launcher