Specs for Contextual Filter

So maybe this would be more appropriate in the views support queue, but I need help setting up the contextual filter to grab the nid to pass to the proximity filter. There seems to be very poor documentation on the contextual filters for D7 and I can't seem to get it right. I'm assuming that I need to provide php code as the default filter, then grab the nid from that, however I've tried:

$node = node_load((int)arg(1));
return $node->nid;

I grabbed that from a forum post I found but no go. I'm thinking that there's just something wrong with that code but I can't figure out what it is.

Any help is greatly appreciated.