With the introduction of CiviAccounts in CiviCRM 4.3 the ground work to allow CiviCRM to cater for Tax against contributions has been laid.
During the week-long sprint in northern California following CiviCon, we've been working through the design and amendments required to complete the implementation of Tax within CiviCRM.
The specification can be found on the wiki
In short we've tried to ensure basic but powerful tax rules are to be built into CiviCRM Core, with additional hooks for users who need to implment specific rules.
Core functionality will include
So out of the box you'll be able to
Using Hooks you'll be able to
Please take a look at the wiki page and provide any feedback.