I just upgraded to Xcode 4 and can't find much documentation on it yet, since it just went gold master. I need to specify a command line argument for testing my application.
The Xcode 3.2 advice doesn't help since everything got moved around.
Command line arguments and environment variables can be set up in the Run section of the scheme editor.
Product -> Edit Scheme... -> Run -> Arguments
(Copy-Paste from the link given by Jano)