When I heard guys on TV talking about how McNamee had saved this stuff for seven years, the first thing I thought about was (for better or worse) how dramatically the Lewinsky scandal changed when she pulled the dress out.
There will be some serious questions about the chain of custody, contamination, etc. I doubt that this evidence would stand up in a court. But that doesn't mean that it won't turn out to be pretty damning to Clemens anyway. He needs to consider what he says under oath very carefully.