Just a quick note to say that as of 1 July the US Copyright Office is allowing online registration. From the Library of Congress NewsNet:
Beginning July 1, 2008, the Copyright Office is offering online registration of claims to copyright. Online registration through the electronic Copyright Office (eCO) is the preferred way to register basic claims for literary works; visual arts works; performing arts works, including motion pictures; sound recordings; and single serials. Advantages of online filing include a lower filing fee; the fastest processing time; online status tracking of your claim; secure payment by credit or debit card, electronic check, or Copyright Office deposit account; and the ability to upload certain categories of deposits directly into eCO as electronic files. To register your claim electronically, go to the Copyright Office website at www.copyright.gov and click on the eCO logo.
Registration is not required to have a copyright in the US, however the work originates in the US you must register in order to file a lawsuit or to claim statutory copyright damages, among other rights. For more, see the FAQ by the Copyright Office: Copyright Registration.