How easily can users of this network be traced by organisations such
as the RIAA? Additionally, if you use Limewire's option to have all
outgoing connections made via an anonymous public proxy server, how
much harder is it for people to trace you?

The easiest way to track users of Gnutella would be with a bot - any
time you share files, the users who download them connect to your IP.
Feasibly, the RIAA has created a bot that has certain keywords built
into it of popular songs, and does searches of the Gnutella network.
If it consistently comes up with the same IP, this is logged by the
bot. RIAA can then issue a subpoena to the ISP of the particular IP
address (found by conducting a search on ARIN WHOIS), and force them
to disclose the customer that was leased the IP address during the
time of alleged infringement.

Routing traffic through proxy servers and blacklisting certain known
RIAA-controlled IP addresses makes you significantly less vunerable to
having your privacy breached by them.

Is it possible to subpoena the owners of proxy servers in these cases?
Do such servers even keep records of who has used them, and for what?