are you taking about the off-yellow box that the browser shows you when you mouse over something? or are you talking about a homemade tooltip solution?
if it's the first, it is the 'title' attribute, so you would want to do :title => 'look at me!' in the html options for the tag. however, i believe IE will show you the alt attribute, so if you do both :alt and :title, you should be safe.