Q: How can I change the taxonomy term selection widget from the default list-box to checkboxes or radio buttons?
A: Just install the 'Taxonomy Super Select' module.

The default taxonomy selection methods provided by drupal may not always be the most suitable for your target website design. For example, a taxonomy vocab that allows multiple selections will be rendered as a multi-select list-box. In the past our clients have found this selection method awkward and have requested that the tax. terms be presented as check boxes instead.

The module - 'Taxonomy Super Select' allows you to render terms as checkboxes for a multi-select vocab and radio buttons for a single select vocab. You just have to install the module, and then edit the settings for the vocab in question (/admin/content/taxonomy). In the settings section called 'Taxonomy Super Select' you will see a list of checkboxes, one for each node type to which this vocab applies. Check those that you want to display as radio buttons or checkboxes and that should do it...