Multi-Site sessions

Hi all,
Im developing an application that runs on multiple physical boxes (and
thus different hostnames). Both of the boxes will run the same
application (connecting to a shared postgres server). What I need is
when someone logs in on box 1 (ie. box1.domain.com), and then traverses
via a link to box 2 (ie. box2.domain.com), he or she will not have to
login again.

That's what the :session_domain key for the session options is for.
Maybe there's more to your setup that you haven't described but if it
is exactly the same code running on the various boxes and you just
happen to have multiple ones because 1 couldn't service your load then
you don't need this at all. You can stick a load balancer in front,
users always go to www.domain.com and the load balancer spreads the
requests out to your various machines (the load balancer can be
anything from specialised hardware to software like haproxy, nginx or
apache).

Fred