Here is an Applescript that does it...
you could remap "spacebar" to the 3rd mouse wheel...but that would not be very good for the rest of your computing experience.
Leopard isn't installed yet, but i'd imagine you can change this in system preferences->keyboard&mouse->keyboard shortcuts
but i'm not entirely sure because i don't have it installed yet.
I wrote an Applescript the does it, use System Preferences->Keyboard & Mouse then for the Mighty Mouse button of your choice select "Other..." and choose this QuickLook Script.app script.
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You can find the Applescript saved as an AppleScript Application Bundle in my .mac public folder.
http://idisk.mac.com/wilhoitm-Public
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All the AppleScript does is simulate a " " keystroke.
tell application "Finder"
activate
tell application "System Events" to keystroke " "
end tell
I am sure there are better solutions but this will suffice for now.
P. S. To make sure this works make sure you click on the "Universal Access" pane in "System Preferences". Click the "Keyboard" tab. At the bottom of the pane is a checkbox setting called "Enable access for assistive devices", click on the checkbox so the setting is enabled.
Marcus