This post turned up in my alerts! , so I joined to respond. (hope the mods let link drops).
Start here:
http://www.podiatry-arena.com/podiatry-forum/showthread.php?t=443
This is a collection of case reports over the last 9 yrs in which a cancer metatasis has resulted in foot pain - in a lot of these cases, the foot pain was the first symptom of the cancer ...its obviously very rare, but very serious when it occurs; there are plenty of research ideas from that (eg what are the red flags that podiatrists need to look out for?, etc)