Last Friday, a judge found that making music available through a file-trading network does constitute copyright infringement, even if that music is ripped from a CD that you bought. (Here's a link to the PDF of the summary judgment.)
According to the Web site Recording Industry vs. The People, which is run by two lawyers representing some defendants in infringment cases brought by the RIAA, this is the second time a judge has made this point, although last time it was included only as a footnote.
Without this legal precedent, the RIAA would have a very hard time suing file-traders. Now, this finding was part of a summary judgment, so it doesn't necessarily count as precedent, but the RIAA has used it to bolster its argument in two other cases. If judges in those cases rule similarly, then the precedent could be established.