Update: I've changed this to put the siproxd.pid file in /tmp which avoids touching the flash memory for the pid file and also avoids the need to mkdir and chmod (which I typoed BTW) and thus allow siproxd to come back up automatically after a reboot.
I bought a WRT54GL today and get openwrt running on it with no hassle. More importantly I got my SIP router thing (a Linksys SPA3102) to work behind it (initially the SIP router was in front of it which is inconvenient in many ways).
To get the SIP thingy working (and also SIP clients on PCs in my home network) I just installed siproxd with
The editing of the config file (like most of useful stuff in this post) comes from this page. I changed the following four entries
the first two were the only ones that required any "figuring out". Then I started siproxd and set it to start at boot time too
After setting up the openwrt box, I just set the outbound proxy for my SIP router and other clients to be my openwrt box and that was that.