Key to the eventual success or failure of Viacom's $1 billion lawsuit against Google/YouTube could be the interpretation of a portion of the Digital Millennium Copyright Act, according to CNET.
The section of the DMCA (PDF) in question deals with the legal liability of hosting companies when it comes to copyrighted material. That provision says hosting companies are not liable for such content on their servers as long as they have safeguards in place and remove it when notified. While YouTube has complied with the second part, the first part could bring into play the question of just how much proactive filtering the site has done.

Viacom attorneys are expected to argue that the "safe harbor" section does not apply to YouTube because the provision covers only hosts that do not directly benefit financially from such content. That could be open to interpretation, though, since it could be argued that though YouTube indirectly benefited from having the clips on the site, it did not attempt to directly benefit by attempting to sell them or otherwise monetizing them.

Napster unsuccessfully attempted to invoke this same "safe harbor" section of the DMCA when it was trying to fend off lawsuits from the record industry.