Seems this subject comes up quite a bit. I have done some limited research, and found this in the 2003 edition of Photographer's Market.
"The Importance of Registration.." "The fact that an image is automatically copyrighted does not mean that it shouldn't be registered. You cannot even file a copyright infringement suit until you've registered your work. Also, without timely registration of your images, you can only recover actual damages-money lost as a result of sales by the infringer plus any profits the infringer earned." "Registration allows you to recover certain damages to which you otherwise would not be legally entitled." Example-attorney fees and court costs and statutory damages.
To register your work, contact the Register of Copyrights, Library of Congress, Washington DC 20559, (202) 707-9100, and ask for Form VA (works of visual art).
Registration costs $30, but you can register large quantities for that fee.
Not sure how this works in other countries, but may be helpful to those of us in the US.