As far as I am aware you will not need a passport until December 31st 2005. For more info check out this site's link from its home page. http://aruba.com/extlinks/travel-state-pdf.html

Expedia just may be getting ready in advance, so they don't have to do a last minute web site deployment or it was too complicated to disclude Aruba from all other international destinations so they didn't bother. I kind of remember expedia doing that when we had our Aruba trip in May 2005. It still wouldn't hurt to get a passport especially if traveling in the future. I happen to have mine and used it in Aruba and elsewhere as ID, my wife used her certificate. Seems things go alot smoother and quicker when you have a passport, they don't double check it against other ID, etc, not that they give you trouble with the certificate either. FYI, Getting in Aruba is alot quicker than getting back, Leave yourself plenty of time when returning to the US.