Google+ loses its cookies in Safari 5

Google Plus is losing its cookies so fast that sometimes it can't keep me logged in for a single click.

In this screenshot, I've logged in. When I click the red "3" for Notifications, the dynamic menu is already logged out, wanting me to Sign In again.

This always happens with Google properties when I'm using Safari. It doesn't happen on Twitter, Flickr, Facebook, Hotmail, Yahoo, etc. Just Google. The problem surfaced with the Safari 4 beta, and has persisted since. It happens whether I have extensions enabled or not, and even if I change cookies from "Block Third Parties" to "Accept Always" (I've left it at Always for weeks now, doesn't help.)