The visibility of the top banner on stackexchange.com seems to be controlled by a cookie instead of being tied to the account. Every time I restart my browser or clear my cookies, I see the "Stack Exchange Q&A communities are different. Here's how:" message.
Can it be permanently hidden for logged in users? While we're at it, can the banner be removed across all sites, even if I haven't joined them? I find myself joining sites just to get rid of the "Here's how it works:" message.