This worked for me and added two tags:
curl --data "api_key=asdf...asdf&api_username=neill&title=this%20one%20has%20two%20tags&raw=and%20it%20is%20twice%20as%20delicious&tags[]=mazda&tags[]=protege" http://localhost:3000/posts.json
So I’m not sure why it isn’t working for you…