After a 5c tax was instituted on plastic bags in Washington DC at the start of the year, the usage dropped from 22.5 million to 3.3 million in THE FIRST MONTH ALONE.

http://voices.washingtonpost.com/dc/2010/03/bag_tax_raises_150000_but_far.html?wprss=dc

This kind of thing is - on a smaller scale - a great example of why carbon pricing/taxation could be so effective if set-up correctly