There are some typos in your documentation for Taggable extension.
In section related to getPopularTags method, if I use your code, like Doctrine::getTable('BlogPost')->getPopularTags(); I get an exception "Unknown method Doctrine_Table::getPopularTags" (with replacing BlogPost by my model name). In effect, it's not the table for my model that I must use but the table for TaggeableTag.
Besides, in your tests, you use Doctrine::getTable('TaggableTag')
I have write a patch to fix these typos. It's attached to this ticket.