ActiveResource Breaking Under FastCGI

I'm developing a web service in rails which uses JSON as the transport
format. I also have a front end rails site that talks to that web
service to retrieve various bits of data. I'm using ActiveResource to
accomplish that.

When running my unit tests for the ActiveResource clients I've written
against a WebBrick server, they all pass fine. However when I run
them against the exact same application code running on FastCGI (on
Dreamhost) it appears that the the save method is partially broken.

The save is successful and my server returns a 201 HTTP status code.
The new entry gets added to the DB. However, the ID attribute of the
entry is blank. When I run this rails application on WebBrick
however, it's populated with the ID attribute of the saved item.

Both servers are running the same version of Ruby and Rails. Anyone
have any idea what could be going on?