The excellent foreigner gem doesn't quite work properly with the new reverse migrations of Rails 3.1. If you specify a column in the add_foreign_key call, the reverse remove_foreign_key_fails. i.e.
add_foreign_key :blogs, :posts, :column => :post_id
This results in:
An error has occurred, all later migrations canceled: wrong number of arguments (3 for 2)
I've forked and fixed this at GitHub.
I've also sent a Pull Request so hopefully this will hit the main repository soon.