A recent question in the newsgroups has set me looking at the way in which Visio's Save as Web addon handles hyperlink tooltips.
The original poster pointed out that in the 2002 version a hyperlink's description text was displayed as the user 'moused-over' a given shape, but that in 2003 and 2007 version the tooltip displays a standard message ('Click to follow hyperlink.') irrespective of the underlying link...