Have attached a picture to show how to setup OpenURL to support Sputnik. As it appears on my machine, Sputnik isn't taking a URL arguement from the commandline, so starting sputnik for a url first (as shown in the prefs picture) - is probably not a good idea. The Arexx section is, however, correct.
Keep in mind that with MUI Alpha and OpenURL, that once the commands are put into the text-gadgets for the prefs that you will have to physically hit the return key to make OpenURL prefs "take."