Not entirely true. Under 17 USC § 411, if you are bringing civil action against someone for copyright infringement, you must have registered your work. However, under 17 USC § 106, owners of copyright have exclusive rights to their works (with some caveats), which doesn't have any baring on their ability to sue someone for copyright infringement.
Where I'm a little hazy is in my reading of 17 USC § 411(c) - I believe that you can only sue for copyright infringement within 3 months of the work being "first transmitted" (which I assume also includes publication), and 48 hours after the work has been "transmitted".