Everything I have heard is that the chances of seeing Whale sharks in July-August-Sept is nearly 100% and 50-50 the rest of the time.
Then I thought, well maybe I'll just go to Utila and see whale sharks another time somewhere else, but the more research I did it seemed like Galapagos in the Summer is pretty much your only reasonably convenient way of having guaranteed encounters. So August '09 it is
Nothing is guaranteed.
I was there last week. Our first attempts at Darwin were unsuccessful for whale sharks, with poor visibility being an issue. They could have been passing reasonably nearby without our knowing it.
We tried again. Our first group of divers splashed in, and our group followed about two minutes later. During those two minutes, our first group got a good view of a whale shark, but we missed it.
On our last attempt, our group was close to giving up and just drifting with the current. A whale shark came up behind us and to the right flank of our group. Our first people saw it as it passed them and tried to alert people. The DM got the message and used his noisemaker to alert everyone. We all swam frantically to the right. I started on the left flank and was about 10 feet behind the rest. Everyone on the right saw it. The three of us on the left did not--the visibility made that much difference.
So, no whale sharks for me.
There was another liveaboard in the area then, and we heard that they had no whale shark sightings at all, so at least most of our group saw one. BTW, it is not easy to watch everyone else's euphoric celebrations when you missed it.