I've been looking for a solution to force RSpec tests to override the attachment_fu storage setting. Since I use s3, it was very inefficient when I'd run my tests because the assets would be saved in s3. Not only does it cost money, but it was very slow.
After spamming #rubyonrails I finally got an answer from _martinS_. He told me to do something like this: http://pastie.org/296895 - and it worked! I hope this helps you guys.