Hi,
This was recently discussed in another post on this board:
http://www.scubaboard.com/t83185-.html
I'm expecting to find them off Utila in the Honduran Bay Islands and/or Ningaloo Marine Park in Australia based on some research I did last year. It also seems like they must migrate in the southwestern Caribbean as there are sightings - and dive ops specializing in Whale Shark encounters in Honduras, Belize and Isla Holbox near Coz.
Some other info and pictures of Whale Sharks can be found at the EcoOcean Whale Shark research site at:
http://www.ecocean.org/index.html. The site somewhat reflects that they're based in Ningaloo though.
If you google Whale Shark Diving Trips, you'll get 142,000 links for review.
We were talking of doing that this summer, but we're now probably filming Mantas off Kona instead.
Steve (from AZScuba)