If you live in the contant paranoia that your site might go down overnight then you might be interested in Google Analytics Intelligence Alerts.

This is a beta addition to the analytics interface and the blub on the help page says:

"An event's significance is determined by comparing the present performance of a given metric with the expected performance of it. Analytics Intelligence is able to calculate the expected performance by taking past performance information into account. If the present and actual performance differs from the expected performance, then Google Analytics will assign a significance rating to it. A higher significance rating is assigned as the difference between the actual performance increases with the expected performance. The significance rating can be seen alongside each metric in the automatic alerts details page, found in the Intelligence reports. "

So this means that you should be alerted if your site traffic stops suddenly, or if you suddenly get hundreds of visitors a day looking for MP3 ringtones (ie you've been hacked). I've yet to try out the custom alerts, but it could be very useful to link the alerts up to an SMS forwarding email account, so that you are instantly notified if your website starts doing strange things. Whether you want to get up at 3am to check your sites still online is up to you.