John brought to my attention a neat little Google Analytics PHP script from Latent Motion. It emails you whenever a website of yours drops in visits, page views, has sales spikes, changes in keyword trends, or in adwords ROI.
Setup requires alerts.php, googleanalytics.class.php (both are included in the zip) and the ID's for your sites (that's the id=[numbers] in the URL of a report).
Customizing the script is easy, you just put your email address, analytics login/password, and so on in their places at the top of the script like so:
12345678, “deniSite2.org” => 1234568, “demoSite3.org” => 1234569);]]>
Then just upload to your server and set it to run as a standard cron job. Pretty easy.
You can get more info, as well as download the scripts at the Latent Motion website.