This was inspired after reading "About Face", and after becoming annoyed with modal dialogue boxes. It displays a text box below a uicontrol for a short period of time, before being removed entirely. Its not a "real" tooltip, perhaps more of a "messagetip".
I tried http://undocumentedmatlab.com/blog/spicing-up-matlab-uicontrol-tooltips/ to try to force the tooltip to appear, but couldn't get it to work. This also doesn't require the use of findjobj, which also takes too long to obtain the java handle.
tooltip(hObject, message)
Brings up a tooltip with the message for the default 4 seconds.

tooltip(hObject, message, displayTime)
Can supply a different length of time to display it for.

tooltip(hObject, message, displayTime, Property/Value pairs)
Formats the box anyway you see fit.

warntooltip(hObject, message,...)
Pre-formatted yellow background.

warntooltip(hObject, message,...)
Pre-formatted red background with white text.