Lawrence Lessig is happy – a US Court of Appeal has provided precedant for the theory behind copyleft ; the basis behind numerous Free Software licenses such as the GNU GPL and specifically in the court ruling, the Artistic License.
The copyleft premise is this: source code has been published in a written form, therefore copyright law restricts your ability to distribute copies of it. A Free Software license grants you an additional license, allowing you to distribute copies; but only if you follow certain conditions.
If you fail to follow the conditions, you are not allowed to claim usage of the Free Software license; therefore full copyright law is in play, and you cannot distribute copies at all.
However, in the US at least, it hasn't been clear whether the conditions attached to these Free Software licenses have actually been “conditions”, and aren't just “contractual covenants”. It makes a big difference when you break them; in the latter case, you are penalised for breaking a contract, and that's all. It would probably result in some monetary damages being due, and they're difficult to assess when the original product was generally available without charges being made. However, given that these clauses are now known to be “conditions”, breaking them means that you have no license at all, and you are infringing copyright – and there is no confusion here regarding money or lack of money.
So, a high-quality precedant from the US legal system supporting the copyleft principal.