How can I prefill taxonomy tags for a new node being created through the node form? I have created a cck node type and associated a tagging taxonomy vocabulary with it. When users create new content, I would like to be able to prepopulate the node form.

For example, if the node type were 'cars' and one term was 'antique', I want to be able to create new instances of cars with the tag 'antique' already populated in the editing form. 

I have gotten as far as creating a path such as 'node/add/auto?tid=3' ... but how do I then take that tid and use it to prepopulate the vocabulary tags field in the auto_node_form?
Have a look at the Prepopulate module.

Read the documentation on how to use it.