I was trying to figure out why curl wasn't able to FTP (in passive mode) through my local SOCKS proxy on OS X. I had configured the .curlrc correctly, and by all rights it should have worked. As it turns out, this is a bug in the version of curl included in OS X 10.4.11 (version 7.13.1). It was only fixed in version 7.18.
This version is new enough that Fink did not have a binary available, but I was able to build it from source once I enabled rsync updating and the unstable repositories.
It was a lot of effort to get the iTunes Companion widget to upload song information through a firewall, but at least I learned something.