Not really that difficult if the database is just designed right.  At least for the destination (shipping address).  I agree more on determining if an item is tax-exempt or not (like food), but the right solution to that is the taxing districts getting rid of those exemptions and simplifying their tax codes so they can be more consistently implemented.

Whether there's 1 tax district or 10 that apply to a particular shipping address, it lends itself to a very simple database query... if we simply administer it sensibly.  

We should just task the Dept of Commerce to provide a database service for the entire country to tap into.  Each state would maintain their own data in the database based on zip+4.