Yes Mitchel, that seems to be a correct assumption.

But I am even more confused now as to where this 20 minutes comes from. This is what I have in the web.config file:

add key="PersistentCookieTimeout" value="10080"

and

forms name=".DOTNETNUKE" protection="All" timeout="60" cookieless="UseCookies"

Nowhere is there a setting for 20 minutes(!?)

If anybody wants to see this in action with the cookies for them selves they could create a userid for the site (here's the registration page) and login and watch the cookie activity (I've been using the cookie extension to the Firebug plugin to do this).

I am utterly perplexed as to what is going on. 

One question that comes to mind is: 

Is it correct behaviour that after you log in (and tick 'remember me') that a DotNetNukeAnonymous cookie continues to exist with a expiration of 20 minues after the log in? This is what I see on my site but NOT what I see here on DotNetNuke.com as i write these words (Firebug is not showing any DotNetNukeAnonymous cookie for my current session on this site, it is instead showing a .DOTNETNUKE cookie with an expiration of 3 Dec).

The plot thickens. Or something like that.