Set-Cookie in rails 2 and rails 3

I noticed that rails 2 sets the session ID cookie on every rails-
served request, whereas rails 3 only sets the session ID when the
session is established.

Anybody know why that might be?

I've had a recurring issue with users losing their session cookie
inexplicably in Safari on a rails 3 app. This never happened with the
rails 2 version, and I just noticed the constant resetting of the
session cookie. This seems like a clue.

Thanks,
Eric