So, according to the DMCA, if copyright holder has any proof whatsoever that the user might be responsible of copyright infringements, the ISP of that user has to hand out the personal details to the copyright holder. Copyright holder is not required to sue the user after they've got the personal details.
Now it seems that Ms. "Jane Doe" and RIAA are already preparing for the actual court fight instead of fighting the subpoena. "Jane Doe" claims that she bought her computer second-hand and that the HDD was full of music and she used Kazaa for listening those tracks and tried to prevent the program from sharing several times without succeeding in that. RIAA is obviously claiming that her P2P usage pattern shows signs of "frequent and significant participant in illegal downloadig and distribution of music".
Anyway, the current issue -- the subpoena case -- relates to the fact that "Jane Doe" claims that handing out her personal details to RIAA violates her privacy and various constitutional rights.
Source: Silicon.com