It is very easy with links and images. One could just specify title="Something" to get a nice tooltip over an element. But somehow I don't see any convenient way to do the same to a button. Seems like HTML itself miss this option at the first place.
But maybe there is at least a handy workaround for that?
Jesse Gumm from nitrogenweb googlegroup shared a nice way to do this
#button{text="Submit",postback=whatever,actions="obj('me').title=\"Some title\""}