Currently there is no option to clear all filters within the Desktop and Reader products (though it is available on Server). I know there have been enhancement requests for this feature (http://community.tableau.com/ideas/1170), but I don't know if this has been added to version 8 or not.
Can someone with the beta chime in and let us know if this is now available in version 8?
If it is not in v8, I recommend voting for this enhancement. The more votes, the more likely it will make a future release.