This one is difficult to judge.
The rule of thumb with typical spiders is that they would only see "text" because it's in the alt attribute. But even that's iffy because they might ignore alt text in an <area> tag.
But tooltips would only appear if they were simulating mouseover events for all the objects in the page. I don't know if any testing has been done on this.
Which, of course, brings us to the true answer, which is that you'd *need* to test a case like this to determine if it is being indexed.