This is an article that talk about that: http://vancouverdesi.com/business/ecb-focus-ecb-mulls-danish-style-deposit-charge-as-lending-prod/ What do you think about? Personally.. it will be a way to animate a bit the forex trading and maybe test the swiss cap of 1.20.
I wrote an article on it in relation to how the practicalities are far removed from the theory. http://morganisteconomics.blogspot.co.uk/2011/09/negative-interest-rates-will-they-work.html