A new bill, The Orphan Works Act of 2008, is currently making its way through congress, and it threatens to take away copyright protection from unregistered works. This includes virtually all open source software.
Essentially, the bill (as I understand it — and I’m not a lawyer) will modify copyright law such that if the owner of a work can not be found by “reasonable search”, anyone can use the work for whatever they want, regardless of the author’s intentions, or the license the work was released under.
This means companies could ignore the GPL, or any other open source license, simply by claiming they couldn’t find the author. If a copyright holder decides to sue, the infringing party just has to show proof that they performed a “reasonable search”.