Back in 2007 I wrote an article for InDesign Magazine on how to add PDF tooltips to an InDesign CS3 file. (Examples of PDFs that incorporate PDF tooltips can be found here and here). In InDesign CS4, Adobe completely reworked the way that buttons are created and edited. The change was for the better, except that in the process, Adobe removed the dialog box that allowed you to enter and edit tooltips. The underlying code for handling tooltips is still in CS4, but the dialog box for viewing, editing and adding tooltips is gone.
I've written a script that adds the functionality for viewing, editing and adding tooltip text to buttons back to InDesign CS4. You can download the "Tooltip" script here.