I found a way, but it requires you to force a re-render of the form. The form must be saved as dynamic for this to work.
Create two fields, TextField1 and TextField2. On the exit event of TextField2, put:
TextField1.assist.getElement("toolTip").value = this.rawValue;
TextField1.presence = "visible";
The second line of code forces the re-render (I tried this using Acrobat 8 ). I find it bizarre that setting the presence of a field that is already visible to visible has any effect, but the resetting of the tooltip doesn't work without the second line. Interestingly, it also works when you set another object on the page to visible. Acrobat seems to re-render the entire form. I'm curious to hear whether this works when using Acrobat or Reader 7 as the client.
Jared Langdon
http://www.jlangdon.ca