Great news for the free software / open source world - in Jacobsen v. Katzer the US Court of Appeals for the Federal Circuit (a leading US IP court) has upheld a free software licence in a way which makes it much easier for the authors of free software to prevent its misuse. (The particular licence is the Artistic licence, but the principles apply across the board).
This is hugely significant as it resolves what has, until now, been a major dispute as to the effect of free software licences in US law.
The mainstream view - that of the proponents of free software (1, 2) - has been that free software licences set conditions on the use of the software. Breach those conditions (e.g. by modifying and then distributing code under a proprietary licence, or by failing to attribute) and the licence evaporates so that you are then infringing the copyright of the author. The full force of copyright law can then come into play - you can, for example, have an interlocutory injunction awarded against you restraining you from using the code.
Some, though, have argued that a free software licence amounts to a general licence to copy, modify, etc. with mere contractual restrictions on what the licensee can do. (E.g.) If true, this would mean that breaching the terms of the licence would merely be a breach of contract, not a breach of copyright. This would, for example, make it more difficult for the author to obtain an injunction against the infringer. It might also cast doubt on the enforceability of free software licences, for example by requiring authors to show that the elements of a contract were present before they could enforce restrictions against infringers.
Jacobsen v. Katzer resolves this argument conclusively in favour of the mainstream view, and holds that while free software licences may also have a contractual element, the restrictions they impose are conditions and not merely contractual restrictions. It also contains a striking judicial endorsement of the objectives and legitimacy of open source / free software generally.
Lessig and Groklaw have more.