It would be nice if something monitored the login server, and notified admins/users that it's down. It could say something like "don't log out or you wont be able to get back in". The best way would be to somehow monitor when people log in, if it got a timeout connecting to minecraft.net, but that might need changes to Bukkit. Otherwise you could fetch http://www.minecraft.net/game/checkserver.jsp and if it gets something unexpected like a timeout or a 503 gateway error, consider that a fail. It will give a login failed or "NOT YET" since you're not properly authenticating of course, but as far as I know every time it was broken, it would time out or give an HTTP error code.
