Rails - Aug 2006 - Sharing sessions across subdomains

Has anyone else experienced this strange behavoir with sharing
sessions across subdomains?

I have in development.rb

ActionController::CgiRequest::DEFAULT_SESSION_OPTIONS.update(:session_domain
=> ''.localhost.com'')

If I create a session at foo.localhost.com then go bar.localhost.com a
session is created (with host localhost.com) and everything works
fine.

But if I then go to www.localhost.com which routes to a different
controller (via the request_routing plugin) another session is created
(with host www.localhost.com).  Now if I navigate back to
foo.localhost.com the second empty session is picked up!

Any ideas?

Thanks,
Zack