A couple of weeks ago I committed some major improvements to the taxation code. There have been so many new features added to Spree these past few weeks that I haven’t had time to write up this stuff until now.
It is now possible to build highly customizable tax rates. You can control which products get taxed (through the Tax Category) and at what rate. You can also vary the rate based on the Zone the order is being shipped to. This allows you to model traditional U.S. sales tax scenarios and even allows for a different tax rate based on the type of product.
For our international friends (and there are a lot of you now) VAT is now supported. There are a few missing pieces but the basic VAT support is there. Please see the new taxation wiki entry for details.