There's been some discussion in GMF about the current login mechanism, in particular dissatisfaction with the user experience of having one's login suddenly evaporate (30 days after your last login) even though one has recently - even a second ago - been logged in and active on site.
Usually I'm heading for a page that requires login (such as GMF's Pending list), so my experience has been to be suddenly faced with the login page. That's fairly self-explanatory (at least to me) as to what happened and what I need to do next.
But if one is heading for a page that doesn't require login, say GMF's or beta's public messages, then the effect is much more disconcerting - you seem to have suddenly been unsubscribed from the group (the button list on the left shrank). If you're paying attention you'll notice that your name disappeared in the upper right, replaced with the Log In and Sign Up links - but that's not the most prominent change.
The consequence is that some group owners get complaints from members who believe they've been unsubscribed without cause, and who generally confound matters in their attempts to re-join the group.
Update the 30-day login period when the user visits the site, rather than only when they go through the login flow.
I presume you wouldn't want to add this to the burden of every page load, but if there's a way to do check it at most once daily per user that ought to be more than adequate.