If you want to uncheck all the checkboxes in a Confluence page with many tasks in it then try this bookmarklet.
Go to Bookmarklet Crunchinator (or similar) to generate a bookmarklet using the contents of src.js below.
See also this feature request.
Update: actually it's not working for me in a bookmarklet, but works fine from Scratchpad in Firefox.
When I use http://bookmarklets.org/maker/ I can get bookmarklet which is not broken, but the script throws a "Task update failed".