Hi -

This took a little bit to locate, but I wasn't getting taxes showing up with a clean install of the latest production release:

In the Tax setup, the first field is used to pick a State. If you use the autofill (ie: start typing Wash... and then select washington) it will default to WA.

However in line 116 of the tax.module, it does this:
drupal_strtoupper($states[$billing->state]) == drupal_strtoupper($rule->realm_value)

That first state (coming from address module) is expecting "WASHINGTON" - the other side of the equation is looking for "WA"

Changing this to:
drupal_strtoupper($billing->state == drupal_strtoupper($rule->realm_value) ...

worked to fix it.