Ha! And I thought I was the only person who wanted to do this! I actually did get my passport stamped in St. Marrten and my roommate thought I was crazy. Guest services will not do this. To do it you must go to immigration at the port. There is a customs office in the industrial area of the port.
When I went in and asked the guy he kinda looked at me strange but I presented my passport. He flipped through it I guess to make sure it was real. He then stamped it in and out. He also wrote on the stamp "In memory of Disney Wonder cruise" and the dates. Hmm. I should take a pic and upload it. I was the first stamp in that passport because it was new at the time.