Coding a token for a profile field for a node's author

I'm trying to code a token module to produce a token I want to use in an automatic URL alias. The token needs to be the content of a profile field for the author of the node whose URL is being set. I've got this pretty figured out as far as getting the token module started (their TokenSTARTER module makes that pretty easy). My problem is - how do I get the nid of the node whose URL is being set?