An important issue here is that downloading music is NOT illegal. Sharing the music is the illegal aspect.
If you get sued, merely claim that you are too stupid to secure your system in order to prevent remote users from having access to your files.
Additionally since many file sharing clients automatically include file sharing as well, if the RIAA accesses your shared files you could in theory counter sue for their accessing your system i a manner "outside of normal use" much like how accessing a system that is infected with a trojan is illegal.
The counter suit would be a bit of a struggle and would actually fall under criminal charges, so good luck with that, but I see it as highly unlikely that the RIAA can prove in court that you are required to be a computer security expert in order to own digital music.
catch
PS. It was ruled a few years back that user requirement site disclaimers and agreements are not binding. However "outside of normal use" is a very real issue and is used for things like prosecuting people for running port/vulnerability scans as well as violating simple to exploit systems like the asp. issue.
