Find or create RESTfully

Hi all,

I am building a system and trying to do it RESTfully using the new and
improved Rails 2.0RC1 and I am not sure how to structure a find or
create action. Basically, I want a user to be able to add a line item
to a list based on vendor. The user has to select the vendor and enter
the product information. When they submit the system will find a list
already exists for that vendor or create a new one, and then adds the
new item to it.

My question is in a RESTful architecture where do I do this? Do I
forward to a create action in case a list does not exist or do I forward
to an update action in case one does?