Here in the US, the RIAA can issue a John Doe subpoena to any ISP demanding to know which of its subscribers was using a given IP address to share copyrighted music at a certain time. Although the RIAA has tried to circumvent this system, it forms a cornerstone of the organization's legal strategy against those it suspects of infringement.
Labels and their legal representatives could have a harder time employing a similar strategy across the pond. An advocate general for the European Court of Justice has issued an opinion stating that ISPs targeted by civil copyright cases in the European Union should not have to identify allegedly-infringing subscribers when pressured to do so by label representatives.
The Spanish version of the RIAA -- Promusicae -- had sued the Telefonica ISP, demanding that it release the identities of allegedly infringing subscribers, but Telefonica refused, claiming that it could only turn over such information as part of a criminal prosecution.
This could be a big hurdle for the record industry P2P litigation strategy in Europe, but we won't know until later this year. According to MarketWatch,
(on marketwatch; via ars technica)