If you go to
http://www.timeanddate.com/worldcloc...omy.html?n=689 you can determine the approximate time of sunset/sunrise in Aruba in May 2008 (i.e. between 6:54 and 7:02 PM depending on the date). That being said I would make sure that you leave enough time to take pictures before the sunset as well (i.e. save your sunset photos for last - you don't want to end up with just sunset photos). Your wedding organizer in Aruba would probably have a better idea when to start the ceremony to make sure you are able to get both pre-sunset/sunset photos.
Having been to a tropical wedding in the past, I would also recommend having your ceremony at the beginning of your trip as opposed to the end of your trip. A wedding can be very stressful and a ceremony at the beginning of the week would allow more time for rest and relaxation.