The log jam seems to be breaking up. I got my invite from corduroysack (++) two days ago -- several hours after he initiated it by including me in a stream message. Since then, every attempt to join took me to the "not yet" page. This morning clicking through took me to the actual sign-up page. My new Google+ membership included 8 invites (bottom of right column). I fired off a couple. One to a person in the same room with me. She got her invite immediately and was able to join without any delay at all.